Q.If A=201λ21−353, then A−1 exists if
(A) λ=2
(B) λ=2
(C) λ=−2
(D) None of these
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Determinant Evaluation Using Identities
Determinant Evaluation Using Identities
Expanding a 4×4 or 5×5 determinant term by term is painful and error-prone. The smarter route is to transform the determinant into an easy form using properties (the "identities") that change its value in a known, controlled way — then read the answer off a triangular matrix.
The geometric intuition
A determinant measures the signed "volume" of the box spanned by the rows in n-dimensional space. Sliding one row parallel to another doesn't change that volume; swapping two rows flips its sign; scaling a row scales the volume. The algebraic identities are just these facts translated into rules.
The three row (or column) operations
- Swap two rows: det→−det (sign flips).
- Scale a row by k: det→kdet (the factor comes out).
- Add a multiple of one row to a different row (Ri→Ri+λRj, i=j): det unchanged.
The identical rules hold for columns. There is also row-wise linearity: if a row is a sum Ri=Ri′+Ri′′, the determinant splits into the sum of two determinants with all other rows fixed.
Row-wise linearity is not det(A+B)=detA+detB — that is false. The splitting works one row at a time.
The strategy
- Use operation 3 to create zeros in a row or column (value unchanged).
- Factor out common factors with operation 2.
- Swap rows if needed to reach upper-triangular form (track the sign change).
- The determinant is then the product of the diagonal entries.
Worked example
det1472583610.
Apply R2→R2−4R1 and R3→R3−7R1 (no change), then R3→R3−2R2: …
Concept: Determinant Evaluation Using Identities — a matrix is invertible iff its determinant is non-zero.
Step 1: Compute det(A) by expanding along the first column:
det(A)=2⋅2153−0⋅λ1−33+1⋅λ2−35
Step 2: Evaluate the 2×2 determinants:
2153=6−5=1,λ2−35=5λ+6 …
detA=5λ+8, which vanishes only at λ=−58; so A−1 exists for λ=−58, matching none of (A)–(C). Correct option: (D).
A−1 exists precisely when detA=0. Expand along the first column (which contains a zero):
detA=22153+1⋅λ2−35=2(6−5)+(5λ+6)=5λ+8.
Check by expanding along the second row:
221−33−521λ1=2(9)−5(2−λ)=5λ+8, …
Method: Testing Invertibility via the Determinant
A square matrix has an inverse exactly when its determinant is nonzero, so any "does A−1 exist" question reduces to computing detA (possibly containing a parameter) and finding the parameter value(s) that make it nonzero.
Steps
Step 1: Recall the invertibility criterion
A−1 exists⟺detA=0
Step 2: Expand detA along the row or column with the most zeros
Keep the parameter symbolic throughout the expansion; this yields detA as a linear (or higher-degree) expression in that parameter.
Step 3: Solve for the excluded (singular) value
Set the expression equal to zero and solve for the parameter — this is the exact value at which the matrix becomes singular and the inverse fails to exist. …
Common Mistakes
Mistake 1: Pattern-matching the answer to "λ=2" without actually computing the determinant
Why it's wrong: The number 2 appears twice in the matrix (positions (1,1) and (2,2)), tempting a guess that the singular condition is λ=2; the real condition, from detA=5λ+8, is λ=−58, which matches none of the given "λ=2"-style options. Correct approach: always compute detA explicitly as a function of the parameter — never infer the singular value from which numbers superficially look connected to the parameter.
Mistake 2: A sign error in the cofactor for the entry holding λ …
Showing the 12 most recent of 32 on this concept.
- AP EAPCET 2025Set eng-2025-05-26-FN1 markMCQQ.If the inverse of −x0x14x1−4x7x0−2x is 20101−2701, then xx+1x+2x+1x+2x+3x+2x+3x+4= (A) 5x (B) x−5 (C) 5x−1 (D) x+5
›Reveal solutionSolution
This tests recognizing a determinant with AP-rows (always zero) and pinning down x from a matrix-inverse condition; the answer is whichever option numerically equals 0 at that x.
Concept and Intuition
A square matrix whose rows form an arithmetic progression (i.e. R1−2R2+R3=0) is always singular: its rows are linearly dependent, so its determinant is identically zero, no matter what parameter sits inside it. Spotting this saves a full cofactor expansion. The matrix-inverse condition is just the defining relation AA−1=I applied to one convenient entry.
Step-by-Step Solution
- Let A=−x0x14x1−4x7x0−2x and A−1=20101−2701.
- Using AA−1=I, multiply row 1 of A by column 1 of A−1: (−x)(2)+(14x)(0)+(7x)(1)=5x. This must equal the (1,1) entry of I, i.e. 1. So 5x=1⇒x=51. (Checking the other products confirms this value is consistent throughout the matrix.)
- Now look at D=xx+1x+2x+1x+2x+3x+2x+3x+4. Apply R1→R1−2R2+R3: each entry becomes x−2(x+1)+(x+2)=0, turning the first row into (0,0,0).
- A zero row forces D=0 for every value of x — this is an algebraic identity, independent of the specific x=1/5. …
- AP EAPCET 2021Set eng-2021-08-23-AN1 markMCQQ.If A(θ)=[isinθcosθcosθisinθ] is a matrix where i=−1, then which of the following is not true (A) detA(π+θ)=detA(−θ) (B) detA(−θ)=detA(θ) (C) det[A(θ)]−1=1 (D) detA(−θ)=−1
›Reveal solutionSolution
The determinant of A(θ) is identically −1, independent of θ; this makes (A), (B), (D) trivially true and exposes (C) as false.
Concept and Intuition
Whenever a matrix's determinant simplifies to a constant using sin2θ+cos2θ=1, every statement that only compares detA at different arguments becomes trivial — the real test is whether the algebra of determinants (like det(M−1)=1/detM) is applied correctly.
Step-by-Step Solution
- Compute detA(θ)=(isinθ)(isinθ)−(cosθ)(cosθ)=i2sin2θ−cos2θ.
- Since i2=−1: detA(θ)=−sin2θ−cos2θ=−(sin2θ+cos2θ)=−1.
- This value is the SAME for every θ (it never even used the sign of the argument), so:
- (A) detA(π+θ)=detA(−θ): both sides are −1. True.
- (B) detA(−θ)=detA(θ): both sides are −1. True.
- (D) detA(−θ)=−1: True. …
- AP EAPCET 2022Set eng-2022-07-05-AN1 markMCQQ.If b and c are non zero real numbers, A=1bcb23c34 and B=0−b−cb0−2c20, then det(A+B)= (A) 3 (B) 1 (C) -1 (D) 0
›Reveal solutionSolution
Adding the symmetric A and skew-symmetric B entrywise cancels the off-diagonal b,c terms below the diagonal, leaving a matrix whose determinant is a fixed number independent of b,c: 3.
Concept and Intuition
A is symmetric and B is skew-symmetric (its transpose is its negative, with zero diagonal). Adding them entrywise, the upper-triangular parts of A+B pick up b+b=2b and c+c=2c, while the lower-triangular parts get b+(−b)=0 and c+(−c)=0 — so A+B becomes upper triangular in its first column, letting the determinant be computed by a clean cofactor expansion that never involves b or c.
Step-by-Step Solution
- A=1bcb23c34, B=0−b−cb0−2c20.
- Add entrywise: (A+B)11=1, (A+B)12=2b, (A+B)13=2c; (A+B)21=b−b=0, (A+B)22=2, (A+B)23=3+2=5; (A+B)31=c−c=0, (A+B)32=3−2=1, (A+B)33=4.
- So A+B=1002b212c54. …
- AP EAPCET 2021Set eng-2021-08-24-AN1 markMCQQ.Let A be a 2×2 matrix with real entries. Let I be the 2×2 identity matrix. Tr(A) denotes the sum of diagonal entries of A. Assume that A2=I. Statement I: If A=I and A=−I then detA=−1. Statement II: If A=I and A=−I then Tr A=0 (A) Statement I is true, statement II is true; statement II is a correct explanation for statement I (B) Statement I is true, statement II is true; statement II is not a correct explanation for statement I (C) Statement I is true, statement II is false (D) Statement I is false, statement II is true
›Reveal solutionSolution
For a real 2×2 matrix with A2=I, A=±I forces eigenvalues {1,−1},
giving detA=−1 (Statement I true) and TrA=0 exactly (so Statement II,
which claims TrA=0, is false).
Concept and Intuition
A2=I says A is an "involution" — applying it twice returns the identity. Its eigenvalues
must satisfy λ2=1, so λ∈{1,−1}. Since x2−1 has two distinct
roots, A is always diagonalizable (no repeated-root Jordan-block subtlety). For a
2×2 matrix there are only three possible eigenvalue multisets: {1,1} (forces
A=I), {−1,−1} (forces A=−I), or {1,−1} — and the hypothesis A=±I
eliminates the first two, pinning down the third exactly.
Step-by-Step Solution
- A2=I⇒ eigenvalues λ satisfy λ2=1⇒λ=±1; since these roots are distinct, A is diagonalizable over R.
- Case eigenvalues {1,1}: a diagonalizable matrix with both eigenvalues equal to 1 is similar to I, and similar-to-I forces the matrix itself to equal I (since PIP−1=I for any P). This case is excluded (A=I).
- Case eigenvalues {−1,−1}: analogously forces A=−I. Excluded (A=−I).
- Remaining case: eigenvalues {1,−1}. This is the only possibility left once A=I,−I is assumed. …
- AP EAPCET 2021Set eng-2021-08-23-FN1 markMCQQ.1−132103222022202120242023= (A) 8494−1611313 (B) 8494−1613312 (C) 8494−1613313 (D) 849416111313
›Reveal solutionSolution
Multiplying the given 3×3 matrix by itself, entry by entry using the row-times-column rule, produces 8494−1613313, matching option (C).
Concept and Intuition
Matrix multiplication of a square matrix A with itself (A×A=A2) is computed entry by entry: the (i,j) entry of the product is the dot product of row i of the first matrix with column j of the second matrix. Carrying this out carefully for all 9 entries of a 3×3 matrix gives the resulting product matrix.
Step-by-Step Solution
Let A=1−13210322. Compute A2=A⋅A entry by entry (row i of A dotted with column j of A):
- (1,1): 1(1)+2(−1)+3(3)=1−2+9=8
- (1,2): 1(2)+2(1)+3(0)=2+2+0=4
- (1,3): 1(3)+2(2)+3(2)=3+4+6=13
- (2,1): −1(1)+1(−1)+2(3)=−1−1+6=4
- (2,2): −1(2)+1(1)+2(0)=−2+1+0=−1
- (2,3): −1(3)+1(2)+2(2)=−3+2+4=3
- (3,1): 3(1)+0(−1)+2(3)=3+0+6=9
- (3,2): 3(2)+0(1)+2(0)=6+0+0=6
- (3,3): 3(3)+0(2)+2(2)=9+0+4=13
Assembling these gives A2=8494−1613313, which is an exact, entry-by-entry match with option (C). …
- AP EAPCET 2022Set eng-2022-07-04-FN1 markMCQQ.If A=[α255−α] and det(A10)=1024, then α= (A) −2 (B) −1 (C) −3 (D) 0
›Reveal solutionSolution
Use det(A10)=(detA)10 to pin down detA=±2, then solve the resulting cubic in α using the actual 2×2 determinant. Answer: α=−3.
Concept and Intuition
For any square matrix, det(An)=(detA)n. So knowing det(A10)=1024=210 tells us (detA)10=210, meaning detA must be a real 10th root of 1024 — since 10 is even, this allows detA=2 or detA=−2 (both give (±2)10=1024). We then compute detA directly from the given matrix in terms of α and match against the candidate integer roots offered.
Step-by-Step Solution
- det(A10)=(detA)10=1024=210⇒detA=2 or detA=−2.
- Compute detA directly: A=[α255−α], so detA=α2(−α)−(5)(5)=−α3−25.
- Case detA=2: −α3−25=2⇒α3=−27⇒α=−3.
- Case detA=−2: −α3−25=−2⇒α3=−23⇒α=−323, not an integer, and not among the answer choices. …
- AP EAPCET 2021Set eng-2021-08-20-AN1 markMCQQ.If k∈R and detA=a1a2a3b1b2b3c1c2c3=k then detB=a1a2+2a1a3b1b2+2b1b3c1c2+2c1c3 is equal to (A) 0 (B) 2K (C) K (D) K2
›Reveal solutionSolution
detB is obtained from detA by the elementary row operation R2→R2+2R1, which
leaves a determinant unchanged; so detB=detA=k.
Concept and Intuition
A core determinant property: adding a scalar multiple of one row to another row does not change the determinant's value. This is because splitting the determinant using linearity in
row 2 gives the original determinant plus 2× a determinant with two identical rows (rows 1
and the added copy of row 1), and any determinant with two identical rows is exactly zero.
Step-by-Step Solution
- detB=a1a2+2a1a3b1b2+2b1b3c1c2+2c1c3.
- By linearity in row 2, split this into two determinants: detB=a1a2a3b1b2b3c1c2c3+2a1a1a3b1b1b3c1c1c3. …
- AP EAPCET 2025Set eng-2025-05-23-AN1 markMCQQ.If α is a real root of the equation x3+6x2+5x−42=0, then the determinant of the matrix α−1α−2α+4α+1α+3α−4α+2α−3α+5 is (A) 90 (B) 120 (C) -105 (D) -135
›Reveal solutionSolution
The cubic has a unique real root α=2 (found by testing small integers and confirming the remaining quadratic has no real roots); substituting into the determinant and expanding gives −105.
Concept and Intuition
When a cubic is asked to have "a real root," it's often meant to be found by the rational root theorem (testing small divisors of the constant term) — here the constant is −42, and 2 divides it. Once found, factor it out and check the resulting quadratic's discriminant to confirm no other real roots exist, so α is unambiguous.
Step-by-Step Solution
- Test x=2 in x3+6x2+5x−42: 8+24+10−42=0. So x=2 is a root.
- Factor: x3+6x2+5x−42=(x−2)(x2+8x+21).
- Discriminant of x2+8x+21 is 64−84=−20<0, so this factor has no real roots — confirming α=2 is the unique real root.
- Substitute α=2 into the matrix:
10635−24−17
- Expand along row 1: …
- AP EAPCET 2024Set eng-2024-05-21-AN1 markMCQQ.a+b+2cccab+c+2aabbc+a+2b= (A) (a+b+c)3 (B) 2(a+b+c)3 (C) 3(a+b+c)3 (D) (a+b+c)
›Reveal solutionSolution
Writing each diagonal entry as s+(off-diagonal-like term) where s=a+b+c reveals the matrix as sI plus a rank-1 correction, whose determinant works out cleanly to 2(a+b+c)3 using the determinant lemma (or row operations).
Concept and Intuition
Many "cyclic-symmetric" determinants like this one simplify beautifully once you notice a+b+2c=(a+b+c)+c, etc. — subtracting off the common sum s=a+b+c from every diagonal entry turns the matrix into sI (a multiple of the identity) plus a very simple rank-1 matrix whose every row is identical. Determinants of sI+rank-1 matrices have a clean closed form.
Step-by-Step Solution
- Let s=a+b+c. Rewrite diagonal entries: a+b+2c=s+c, b+c+2a=s+a, c+a+2b=s+b.
- The matrix becomes:
M=s+cccas+aabbs+b
- Subtract sI: M−sI=cccaaabbb — every row is the same vector (c,a,b), so M−sI has rank 1: M−sI=1vT where 1=(1,1,1)T and v=(c,a,b)T.
- So M=sI+1vT. The determinant lemma (matrix determinant lemma) gives: det(sI+1vT)=s3+s2(vT1) …
- AP EAPCET 2022Set eng-2022-07-04-FN1 markMCQQ.For i=1,2,3 and j=1,2,3. If ai2+bi2+ci2=1, aiaj+bibj+cicj=0, ∀i=j and A=a1b1c1a2b2c2a3b3c3 then det(AAT)= (A) 0 (B) 1 (C) −1 (D) 3
›Reveal solutionSolution
The conditions given say the rows of A form an orthonormal set, which is precisely the condition AAT=I; so its determinant is 1. Answer: 1.
Concept and Intuition
For a matrix A with rows R1,R2,R3, the (i,j) entry of AAT is exactly the dot product Ri⋅Rj. The problem states ai2+bi2+ci2=1 (each row is a unit vector) and aiaj+bibj+cicj=0 for i=j (distinct rows are perpendicular). Together these say the rows are orthonormal — which is the defining property of an orthogonal matrix, for which AAT=I.
Step-by-Step Solution
- Write (AAT)ij=Ri⋅Rj=aiaj+bibj+cicj.
- For i=j: (AAT)ii=ai2+bi2+ci2=1 (given).
- For i=j: (AAT)ij=aiaj+bibj+cicj=0 (given).
- So AAT=I3, the 3×3 identity matrix.
- det(AAT)=det(I3)=1.
Common Mistakes …
- AP EAPCET 2025Set eng-2025-05-23-FN1 markMCQQ.2311+131/31+1/231/91+1/431/271+…∞= (A) 0 (B) 21 (C) −21 (D) −1
›Reveal solutionSolution
Expand each 2×2 determinant, recognise two independent geometric series in the result, and sum them separately. Answer: (C).
Concept and Intuition
Every term in the sum is a 2×2 determinant with a fixed bottom row (31), so a3b1=a−3b splits linearly into an a-part and a b-part. Once each part is recognised as its own geometric progression, the infinite sum is just the difference of two standard geometric series sums, 1−rfirst term.
Step-by-Step Solution
- General term: an3bn1=an⋅1−bn⋅3=an−3bn.
- First-column values across the given terms: 2,1,21,41,… — a geometric sequence with first term 2 and ratio 21: an=2(21)n−1.
- Second-column values: 1,31,91,271,… — geometric with first term 1 and ratio 31: bn=(31)n−1. …
- AP EAPCET 2023Set eng-2023-05-17-FN1 markMCQQ.If f(x)=1006+xx−32x−436+x23x2−278x2−32, then x→1limf(−x)f(x)= (A) 2 (B) −1 (C) 0 (D) 1
›Reveal solutionSolution
Expanding the determinant reveals f(x)=2(x−1)(x−2)(x−3), a factor that vanishes at x=1, so the ratio f(x)/f(−x) has a zero numerator and finite nonzero denominator there — limit 0.
Concept and Intuition
Determinants with a column like (1,0,0)T expand trivially (cofactor of the top-left entry only), collapsing a 3×3 determinant into a 2×2 one. Recognising the resulting expression as a product of simple linear factors (rather than grinding through raw polynomial expansion) makes the limit evaluation immediate.
Step-by-Step Solution
- Expand along column 1 (entries 1,0,0): f(x)=1⋅x−32x−43x2−278x2−32=(x−3)(8x2−32)−(3x2−27)(2x−4).
- Factor: 8x2−32=8(x−2)(x+2), 3x2−27=3(x−3)(x+3), 2x−4=2(x−2).
- f(x)=8(x−3)(x−2)(x+2)−6(x−3)(x+3)(x−2)=(x−3)(x−2)[8(x+2)−6(x+3)]=(x−3)(x−2)(2x−2)=2(x−1)(x−2)(x−3).
- f(−x)=2(−x−1)(−x−2)(−x−3)=2⋅(−1)3(x+1)(x+2)(x+3)=−2(x+1)(x+2)(x+3). …
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