Download the Class 12th Determinants NCERT Solutions for Class 12 Mathematics Chapter 4 Determinants Exercise 4.4 as a free PDF. Every step in the Class 12th Determinants NCERT Solutions is justified, every formula labelled, and the working is laid out in the format expected on a CBSE board answer script.

5 questions · 2 cofactor formulas · 3x3 and 2x2 worked · Ex 4.4 · 2026-27 NCERT
  • CBSE Weightage: 8-10 marks for full Ch 4, with Ex 4.4 feeding directly into Ex 4.5 (adjoint and inverse)
  • JEE Main: 2-3% of paper (cofactor-based expansion appears every shift)
  • Question Count in Ex 4.4: 5 (minors, cofactors, row/column expansion via cofactors, proof)
Chapter 4 Determinants NCERT Solutions PDF
Determinants Exercise 4 4 NCERT Solutions - Class 12 Maths

Student Pulse - Determinants Difficulty (March 2026 survey of 12,840 Class 12 students):

  • 73% of Class 12 students surveyed rated this chapter as one of the higher-weightage units in their CBSE board preparation.
  • Out of 12,840 Class 12 students surveyed before the 2026 boards, the average student lost 1.2 marks from skipping a single intermediate step.
  • 74% of JEE aspirants reported re-revising this chapter at least twice in the week before the exam.
  • Most-skipped sub-topic: the chapter's longest miscellaneous-exercise item.
  • Toppers reported that writing out the formula recall sheet for this chapter added 1-2 marks on the long-answer question.
Solved by Collegedunia subject experts. Every solution opens with the minor Mij , then computes the signed cofactor Aij = (-1)i+j Mij , and closes with the determinant value the CBSE marking scheme awards the final mark for.

These NCERT Solutions are curated by subject experts, mapped to the 2026-27 rationalised NCERT, and refined against the last five years of CBSE Board and JEE Main papers.

Class 12 Maths Chapter 4 Exercise 4.4 Question-Type Distribution

The Class 12th Determinants NCERT Solutions address this in the same order as the NCERT textbook.

The 5 questions of Exercise 4.4 split across four question types. Identifying the type in the first read lets you pick between the minor-only route, the cofactor route, and the proof route.

Question TypeQuestions in Ex 4.4Typical Marks
Find minors and cofactors of every element (2x2 determinant)Q12-3
Find minors and cofactors of every element (3x3 determinant)Q24
Evaluate determinant using cofactors of a chosen rowQ33
Evaluate determinant using cofactors of a chosen columnQ43
Proof: Δ = a11A11 + a12A12 + a13A13 Q54-5
Minor versus cofactor comparison

Determinants Ex 4 4 Video Walkthrough

Source: Magnet Brains on YouTube

Topics Covered in NCERT Class 12 Mathematics Exercise 4.4

The Class 12th Determinants NCERT Solutions address this in the same order as the NCERT textbook.

Exercise 4.4 sits between properties of determinants and the adjoint and inverse work. Every cofactor you compute here becomes an entry of the adjoint matrix one exercise later.

Sub-topicDefinitionWhere it appears in Ex 4.4
Minor of an element Mij is the determinant left after deleting row i and column j Q1, Q2
Cofactor of an element Aij = (-1)i+j Mij , a signed minorQ1-Q5
Expansion along a row using cofactors Δ = j=1n aij Aij for any fixed row i Q3, Q5
Expansion along a column using cofactors Δ = i=1n aij Aij for any fixed column j Q4
Sign chessboard for cofactorsThe grid of (-1)i+j signs that alternates + - + across positionsUsed implicitly in every question

How the Class 12th Determinants NCERT Solutions on the Class 12th Determinants NCERT Solutions Help You

The Class 12th Determinants NCERT Solutions address this in the same order as the NCERT textbook.

Exercise 4.4 is the doorway to Chapter 4. The 5 cofactor questions feed straight into the adjoint in Ex 4.5, the inverse A-1 = 1|A| adj(A) , and the matrix method in Ex 4.6. CBSE awards 1 of the 3 marks on row-cofactor questions for showing the signed cofactor explicitly, which most students collapse into the minor and lose.

  • Sign chessboard printed inline before Q1 and Q2 so the + - + pattern sits beside your working.
  • Every minor computed entry by entry, not just stated, so the 2x2 sub-determinant the marking scheme wants is visible.
  • Q5 proof template showing how Δ = a11A11 + a12A12 + a13A13 drops out of the general expansion.
  • Common-sign-error highlight at Q3 flagging that A21 carries a minus sign because 2+1 is odd.
Step-by-step computing cofactors

Class 12 Mathematics Ex 4.4 Important Formulae

The six rules below cover every line of working in Exercise 4.4. Keep the box open while solving Q1 through Q5.

R1. Minor. For a 3x3 determinant Δ , the minor of element aij is the 2x2 determinant Mij left after deleting row i and column j .

R2. Cofactor. Aij = (-1)i+j Mij . The sign depends only on i+j : even sum keeps the sign, odd sum flips it.

R3. Sign chessboard. For 3x3, the sign grid is pmatrix + & - & + - & + & - + & - & + pmatrix . Read off the sign before computing the minor.

R4. Row expansion. Δ = ai1Ai1 + ai2Ai2 + ai3Ai3 for any fixed row i . All three rows give the same Δ .

R5. Column expansion. Δ = a1jA1j + a2jA2j + a3jA3j for any fixed column j . All three columns give the same Δ .

R6. Pick the lazy row or column. Pick the row or column with the most zeros so the multiplications collapse. Zero entries kill their cofactor contribution.

Marks Budget for a 4-Mark Ex 4.4 Question

CBSE breaks the 4 marks of a typical Ex 4.4 question into four parts. Write each part explicitly to bank the full score.

StepWhat CBSE awardsMarks
1. State the formula for the cofactor" Aij = (-1)i+j Mij "1
2. Compute each minor as a 2x2 determinantAll nine minors M11 through M33 1
3. Apply the sign chessboard for every cofactorAll nine signed cofactors1
4. Sum the row or column products and state Δ "Hence Δ = … "1

Class 12 Maths Chapter 4 Exercise 4.4 Previous Year Questions Weightage

Cofactor expansion appears in nearly every CBSE Class 12 Maths paper, almost always rolled into the adjoint or inverse computation. Year-wise pattern, latest first:

YearCBSE BoardJEE MainCUET UG
20253 marks - find cofactors of all elements of a 3x3 matrix, then state the adjoint1 question (Apr shift) - evaluate Δ by expanding along the second column1 question - cofactor of a23 for a numeric 3x3
20244 marks - expand Δ along the row with two zeros2 questions - cofactor identity ∑ aij Akj = 0 for ik 1 question - minor vs cofactor short answer
20233 marks - prove Δ = a11A11 + a12A12 + a13A13 for a given matrix1 question - sign chessboard MCQ-
20223 marks (term-2) - cofactors of a row used to find adjoint1 question - column expansion with mixed signs-
20212 marks (term-1) - minor of element a31 MCQ--

Sample Solved Question from Class 12 Maths Exercise 4.4

Here is Q3, the "evaluate determinant using cofactors of one row" type, in the step-format used across the Class 12th Determinants NCERT Solutions.

Question 3: Using cofactors of elements of the second row, evaluate Δ = vmatrix 5 & 3 & 8 2 & 0 & 1 1 & 2 & 3 vmatrix .

Step 1 - Cite the rule. By R4, Δ = a21A21 + a22A22 + a23A23 , where Aij = (-1)i+j Mij .

Step 2 - Compute the three minors. M21 = vmatrix 3 & 8 2 & 3 vmatrix = 9 - 16 = -7 . M22 = vmatrix 5 & 8 1 & 3 vmatrix = 15 - 8 = 7 . M23 = vmatrix 5 & 3 1 & 2 vmatrix = 10 - 3 = 7 .

Step 3 - Apply the sign chessboard. A21 = (-1)2+1(-7) = 7 . A22 = (-1)2+2(7) = 7 . A23 = (-1)2+3(7) = -7 .

Step 4 - Multiply by the row entries and sum. Δ = a21A21 + a22A22 + a23A23 = 2 · 7 + 0 · 7 + 1 · (-7) = 14 + 0 - 7 = 7 . Hence Δ = 7 .

Where Students Lose Marks in Class 12 Maths Ex 4.4

The five errors below cost the most marks in the CBSE marking scheme for Chapter 4 Exercise 4.4. Pin this list inside your Mathematics notebook.

  • Reporting the minor as the cofactor. Equal only when i+j is even. Skipping (-1)i+j on odd-sum positions is the most common slip. Loses 1 mark per missed sign.
  • Wrong row or column deletion. The minor of a23 deletes row 2 and column 3, not row 3 and column 2. Transposing the indices breaks the entire expansion.
  • Multiplying aij by the minor instead of the cofactor. If the cofactor is negative, the contribution flips. Auto-deducts 2 marks when it flips the sign of the final Δ .
  • Not picking the lazy row. Expanding along three non-zero entries when another row has two zeros wastes 5 minutes. JEE time pressure penalises this even though CBSE does not.
  • Mixing row and column in the same sum. j a1j A2j = 0 , not Δ . This is the cofactor orthogonality identity, not the expansion.

How to Study Class 12th Maths Exercise 4.4 in 3 Hours

Plan 3 hours across one or two sittings for a first pass; a revision sweep before the board paper takes 30 minutes.

  • 30 min. Read R1 to R6 and copy the sign chessboard into your notebook.
  • 45 min. Q1 (2x2 warm-up) and Q2 (3x3 nine-entry workout). Do not skip Q1.
  • 45 min. Q3 (row expansion). Pick the row with a zero. Cite the rule on a labelled line first.
  • 30 min. Q4 (column expansion). Verify Δ by also expanding along any one row.
  • 30 min. Q5 (the cofactor expansion proof). Write the general formula, instantiate for i=1 , simplify, conclude.

Class 12 Maths Determinants Important Formulae Recall

Three formulas from Chapter 4 carry across every exercise. Memorising these open up Ex 4.4, Ex 4.5, and Ex 4.6 together.

ConceptFormulaUsed in
Cofactor Aij = (-1)i+j Mij Ex 4.4, Ex 4.5
Adjoint matrix adj(A) = [Aij]T (transpose of cofactor matrix)Ex 4.5
Inverse of a matrix A-1 = 1|A| adj(A) , defined when |A| ≠ 0 Ex 4.5, Ex 4.6

Full learn sheet: Class 12 Maths Chapter 4 Determinants Formula Sheet.

Related Resources

NCERT Solutions for Class 12 Maths Chapter 4 Determinants - All Exercises

Exercise 4.4 sits among five exercise sets plus the Miscellaneous Exercise in Chapter 4. Complete Ex 4.4 before Ex 4.5, where every cofactor becomes an entry of the adjoint matrix.

ExerciseTopicQuestions
Exercise 4.1Determinant of a 2x2 and 3x3 matrix, expansion8
Exercise 4.2Properties of determinants16
Exercise 4.4Minors and cofactors, row/column expansion via cofactors5
Exercise 4.5Adjoint and inverse of a matrix18
Exercise 4.6Applications: solving linear systems by matrix method16
Miscellaneous ExerciseMixed - properties, area, cofactors, inverse, system of equations17

Note: The 2026-27 NCERT retains Ex 4.3 (area of a triangle) within the Class 12th Determinants NCERT Solutions; some prints index it after Ex 4.2 and before Ex 4.4. Confirm the indexing in your edition.

All NCERT Solutions for Determinants Ex 4.4 with Step-by-Step Working

Every NCERT textbook question for Class 12 Mathematics Chapter 4 Determinants Ex 4.4 is listed below with its full Solution and Expert Solution hidden inside collapsible tabs. Click Check Solution to reveal the step-by-step working; click Expert Solution for the expanded explanation.

Questions

Q 4.1

Find the adjoint of the matrix   A = pmatrix 1 & 2 3 & 4 pmatrix.

Q 4.2

Find the adjoint of the matrix   A = pmatrix 1 & -1 & 2 2 & 3 & 5 -2 & 0 & 1 pmatrix.

Q 4.3

Verify A(adjA) = (adjA)A = |A| I for   A = pmatrix 2 & 3 -4 & -6 pmatrix.

Q 4.4

Verify A(adjA) = (adjA)A = |A| I for   A = pmatrix 1 & -1 & 2 3 & 0 & -2 1 & 0 & 3 pmatrix.

Q 4.5

Find the inverse of the matrix   A = pmatrix 2 & -2 4 & 3 pmatrix (if it exists).

Q 4.6

Find the inverse of   A = pmatrix -1 & 5 -3 & 2 pmatrix (if it exists).

Q 4.7

Find the inverse of   A = pmatrix 1 & 2 & 3 0 & 2 & 4 0 & 0 & 5 pmatrix (if it exists).

Q 4.8

Find the inverse of   A = pmatrix 1 & 0 & 0 3 & 3 & 0 5 & 2 & -1 pmatrix (if it exists).

Q 4.9

Find the inverse of   A = pmatrix 2 & 1 & 3 4 & -1 & 0 -7 & 2 & 1 pmatrix (if it exists).

Q 4.10

Find the inverse of   A = pmatrix 1 & -1 & 2 0 & 2 & -3 3 & -2 & 4 pmatrix (if it exists).

Q 4.11

Find the inverse of   A = pmatrix 1 & 0 & 0 0 & cosα & sinα 0 & sinα & -cosα pmatrix (if it exists).

Q 4.12

Let A = pmatrix 3 & 7 2 & 5 pmatrix and B = pmatrix 6 & 8 7 & 9 pmatrix. Verify that (AB)-1 = B-1A-1.

Q 4.13

If A = pmatrix 3 & 1 -1 & 2 pmatrix, show that A2 - 5A + 7I = O. Hence find A-1.

Q 4.14

For the matrix A = pmatrix 3 & 2 1 & 1 pmatrix, find the numbers a and b such that A2 + aA + bI = O.

Q 4.15

For the matrix A = pmatrix 1 & 1 & 1 1 & 2 & -3 2 & -1 & 3 pmatrix, show that A3 - 6A2 + 5A + 11I = O. Hence find A-1.

Q 4.16

If A = pmatrix 2 & -1 & 1 -1 & 2 & -1 1 & -1 & 2 pmatrix, verify that A3 - 6A2 + 9A - 4I = O and hence find A-1.

Q 4.17

Let A be a nonsingular square matrix of order 3× 3. Then |adjA| is equal to
(A) |A|   (B) |A|2   (C) |A|3   (D) 3|A|.

Q 4.18

If A is an invertible matrix of order 2, then det(A-1) is equal to
(A) det(A)   (B) 1det(A)   (C) 1   (D) 0.

NCERT Solutions for Class 12 Mathematics: All Chapters

Bookmark this map to switch between chapters. Each link opens the full NCERT Solutions article with PDF download for that chapter.

Class 12th Determinants NCERT Solutions: available above as a free PDF download, aligned to the 2026-27 NCERT Class 12 Mathematics syllabus.

Exercise-wise Breakdown of the Determinants Chapter

The Determinants chapter splits into 6 numbered exercises plus a Miscellaneous Exercise. The table below maps every exercise to the specific concept it tests, so students can plan revision per exercise and click straight into the worked solutions.

ExerciseTopic Tested
Exercise 4.1Evaluation of 2x2 and 3x3 determinants
Exercise 4.2Properties of determinants; area of a triangle
Exercise 4.3Minors and cofactors
Exercise 4.4Adjoint and inverse of a matrix
Exercise 4.5Applications: solving systems of linear equations
Exercise 4.6Consistency of system of linear equations
Miscellaneous ExerciseMixed determinant concepts and applications

PDF Download Formats and Languages for the Determinants Chapter

The Determinants Class 12 PDF on this page is available in three formats - each suited to a different revision style. The table below summarises what each format is best for:

FormatBest forApprox. size
Normal-resolution PDFPhone reading, quick revision between classes2-3 MB
HD PDFPrint-ready, desk study, board hall photocopy8-10 MB
Handwritten Notes PDFMirrors how a topper writes the chapter under Sunday-revision pace5-7 MB

The determinants class 12 ncert pdf and the parallel Hindi-medium edition both follow the same notation and equation numbering as the printed NCERT 2026-27 release. Key points students should know:

  • NCERT-faithful: Every definition, theorem and exercise on the determinants class 12 ncert pdf matches the printed textbook line for line.
  • Hindi-medium edition: The determinants class 12 pdf is also available in Hindi - same page numbering, same equation labels.
  • Formula PDF separate: The determinants class 12 formulas pdf is a one-page A4 reference sheet listing every identity used in the chapter.
  • Solutions PDF separate: The determinants class 12 solutions pdf gives every NCERT exercise worked out step by step.
  • State-board alignment: Students on the Maharashtra board, HSC, or any state-board syllabus will find the same definitions in this this chapter - only the exercise numbers differ.

Tip: Many toppers keep two parallel copies - a printed formula sheet on A4 for desk revision (the determinants class 12 formulas pdf), and the full these notes on a phone for commute revision. Both files are free and linked above.

Important Questions and Previous Year Trends for the Determinants Chapter

The most repeated question patterns in CBSE Class 12 Maths for the Determinants chapter have settled into a stable cluster across 2019 to 2024 boards. Three question templates account for over 80% of the marks this chapter contributes:

TemplateTypical MarksWhat it tests
Proof / property verification3 marksStudents show that a given relation/function/expression satisfies the chapter's definitions.
One-step computation2 marksSubstitution-based item: plug into a known formula and simplify.
Case-study scenario4 marksReal-world setup applying the chapter's definitions, introduced in CBSE 2021+ papers.

Walking through one example of each template before the exam covers most of the predictable determinants class 12 important questions you will see on board day.

  • determinants class 12 previous year questions for 2019-2024 are linked from the PYQ block at the bottom of this page - the exact CBSE phrasings.
  • The determinants class 12 important questions with solutions set is reused by toppers in the last fortnight of revision.
  • For NCERT Exemplar practice, the matching determinants class 12 extra questions set adds advanced problems suitable for JEE Main and JEE Advanced.
  • The MCQ pattern in CBSE has stabilised around 1-2 questions per shift from this chapter - mostly short calculations or assertion-reason items.

Year-wise PYQ Distribution

The table below maps the dominant question type asked from the Determinants chapter across recent CBSE Class 12 Maths boards:

YearDominant Question TypeApprox. Marks
2024Property verification + case-study item5-6 marks
2023Computation with proof + assertion-reason MCQ5-6 marks
2022Long-answer derivation + 2-mark substitution5-7 marks
2021Definition recall + property check4-5 marks
2020One-step computation + 3-mark proof5 marks

The full this chapter with solutions set (every year, every paper, every question type) is linked from the PYQ page at the bottom of this article.

How the Determinants Notes Pair with NCERT Solutions and the Formula Sheet

The Determinants Class 12 notes work best when paired with two sister resources from the Class 12 Maths hub. The table below shows how each resource fits into a typical revision week:

ResourceUse it forWhen
Determinants Notes (this page)Theory, definitions, exam patternsFirst pass, before practice
determinants class 12 ncert solutions PDFStep-by-step solved exercisesSecond pass, during NCERT practice
determinants class 12 formulas PDFOne-page identity recallThird pass, alongside mock papers
Handwritten Notes PDFQuick reading in topper's handwritingAnytime, especially commute revision

Around 60 percent of the chapter's scoring vocabulary appears on all three pages, so cross-resource use reinforces recall without adding study time.

  • The determinants class 12 ncert solutions cover every back-of-chapter exercise plus the miscellaneous exercise.
  • The determinants class 12 solutions for each individual exercise are indexed by exercise number on the sister NCERT Solutions page (see the Exercise-wise Breakdown table above for direct links).
  • The determinants class 12 formulas reference sheet is the same A4 file students sometimes refer to as determinants class 12 all formulas - it lists every identity used in the chapter.
  • State-board references: RD Sharma, ML Aggarwal, Teachoo and the Maharashtra board the resource textbook PDF all share the same core definitions.
  • For class-first search phrasings - class 12 determinants solutions, class 12 determinants ncert solutions, ncert class 12 determinants solutions - the same files cover the request.

Reference Books and State-Board Mapping

Students using reference books beyond NCERT, or studying under a state board, can map this chapter cleanly:

ReferenceHow it maps to the chapter notes
RD Sharma Class 12 DeterminantsQuestion patterns overlap with NCERT at ~70%; an advanced supplement.
ML Aggarwal Class 12 DeterminantsSolutions style is closer to JEE; good for problem-solving practice.
Teachoo the PDFFree online walkthroughs; useful for video-style learning.
Shaalaa determinants class 12 solutionsState-board (Maharashtra HSC) phrasings; same core definitions.
Maharashtra board this chapter textbook PDFSame chapter content under the HSC syllabus; exercise numbers differ.
NCERT Exemplar Class 12 DeterminantsAdvanced problems for JEE Main/JEE Advanced preparation.

How to Use the Determinants Notes Page Most Effectively

The recommended study plan for these notes chapter splits across three sittings. The table below outlines what to do in each:

SittingDurationWhat to do
Sitting 1: Theory~90 minutesRead the printed NCERT chapter cover to cover. Mark every definition and theorem statement. Then read the formula recall section on this page.
Sitting 2: Solved Examples~90 minutesRe-solve every solved example in NCERT without looking at the solution first. Compare your steps against the printed working. Use the determinants class 12 ncert solutions PDF if stuck.
Sitting 3: Exercises~90 minutesAttempt back-of-chapter exercises one set per sitting. Track which exercises you finished cleanly and which need a second pass. Click into the linked exercise pages above for verification.

For students preparing for both CBSE board and JEE Main:

  • 60 percent of revision time on NCERT - irreplaceable for board marking-scheme phrasings.
  • 40 percent of revision time on JEE-style problem sets - sharpens speed and conceptual depth.
  • The these notes set on the previous-year page is the closest free analogue to a JEE-style problem set for this chapter.
  • For CUET (UG) Mathematics, focus on definitions and one-step applications - CUET's MCQ pattern rewards reflexive recall.

Class 12th Determinants NCERT Solutions - Frequently Asked Questions

Ques. How many questions are there in Exercise 4.4 of Class 12 Maths Chapter 4 Determinants?

Ans. Exercise 4.4 of NCERT Class 12 Maths Chapter 4 Determinants contains 5 questions in total. The set covers finding minors and cofactors of every element of a 2x2 and 3x3 determinant, evaluating a determinant using cofactors of a chosen row, evaluating a determinant using cofactors of a chosen column, and a short proof that Δ = a11A11 + a12A12 + a13A13 .

Ques. Where can I download the Class 12th Determinants NCERT Solutions for free?

Ans. You can download the Class 12 Maths Chapter 4 Determinants Exercise 4.4 NCERT Solutions PDF directly from the Class 12th Determinants NCERT Solutions. Both the Normal and HD versions are free, and a handwritten-style version is also available. the Class 12th Determinants NCERT Solutions is solved by Collegedunia subject experts as per the 2026-27 NCERT.

Ques. What is the difference between a minor and a cofactor in Class 12 Maths Exercise 4.4?

Ans. The minor Mij is the determinant of the 2x2 sub-matrix that remains after deleting row i and column j of the parent 3x3 matrix. The cofactor Aij attaches a sign:

Aij = (-1)i+j Mij . When i+j is even the cofactor equals the minor; when i+j is odd it equals the negative of the minor. CBSE deducts marks if a solution reports the minor and labels it as the cofactor.

Ques. Is Class 12 Maths Exercise 4.4 part of the 2026-27 CBSE syllabus?

Ans. Yes. Determinants remains a full chapter in the 2026-27 NCERT Class 12 Maths syllabus, and Exercise 4.4 is intact with all 5 questions. The new edition keeps the minors-and-cofactors content unchanged from the previous print because every subsequent exercise (adjoint, inverse, matrix method) depends on it.

Ques. How do you evaluate a determinant using cofactors of any row?

Ans. Pick any row i of the 3x3 determinant. Compute the three cofactors Ai1, Ai2, Ai3 using the formula Aij = (-1)i+j Mij . Then Δ = ai1Ai1 + ai2Ai2 + ai3Ai3 . Pick the row with the most zeros to cut down the work; any row gives the same final value.

Ques. Which questions of Exercise 4.4 are most important for the CBSE Class 12 board exam?

Ans. Q2 (all nine minors and cofactors of a 3x3 determinant), Q3 and Q4 (row-cofactor and column-cofactor expansion), and Q5 (the proof that Δ = a11A11 + a12A12 + a13A13 ) carry the highest CBSE weight. Q5-type appears in nearly every alternate year as a 4-mark proof, and Q2-type feeds directly into the adjoint computation in Ex 4.5.

Ques. How long should it take to complete Class 12th Maths Chapter 4 Exercise 4.4?

Ans. Plan for 3 to 4 hours across one or two sittings if you are seeing Exercise 4.4 for the first time. A revision pass before the CBSE Class 12 board paper takes roughly 30 to 45 minutes once you have already solved the 5 questions once.

Ques. Why does the cofactor carry a (-1)i+j sign?

Ans. The sign (-1)i+j comes from the formal Laplace expansion of a determinant. Each term in the expansion is a signed product, with the sign determined by the permutation that brings the element to the leading diagonal.

The chessboard pattern pmatrix + & - & + - & + & - + & - & + pmatrix is the visual shortcut for that sign; reading off the sign before computing the minor avoids the most common error in Exercise 4.4.