If the system of linear equations 3x + y + beta z = 3 2x + alpha y - z = -3 x + 2y + z = 4 has infinitely many solutions, then the value of 22beta - 9alpha is:

Solution & Explanation

### Related Formula Cramer's Rule for infinite solutions specifies that the main determinant and all component determinants must vanish: Delta = 0 quad textand quad Delta_1 = Delta_2 = Delta_3 = 0 ### Core Logic Set the key system determinants to zero to form equations linking alpha and beta, then isolate the constants. ### Step 1: Set Main Determinant to Zero Delta = beginvmatrix 3 & 1 & beta \\ 2 & alpha & -1 \\ 1 & 2 & 1 endvmatrix = 0 Expand along the first row: 3(alpha + 2) - 1(2 + 1) + beta(4 - alpha) = 0 3alpha + 6 - 3 + 4beta - alphabeta = 0 implies 3alpha + 4beta - alphabeta + 3 = 0 quad dots (1) ### Step 2: Set Subsidiary Determinant to Zero Using Delta_3 = 0 by substituting the constants vector into the third column: Delta_3 = beginvmatrix 3 & 1 & 3 \\ 2 & alpha & -3 \\ 1 & 2 & 4 endvmatrix = 0 Expand along the first row: 3(4alpha + 6) - 1(8 + 3) + 3(4 - alpha) = 0 12alpha + 18 - 11 + 12 - 3alpha = 0 implies 9alpha + 19 = 0 implies alpha = -frac199 ### Step 3: Solve for Beta and Final Expression Substitute alpha = -frac199 into equation (1): 3left(-frac199right) + 4beta - left(-frac199right)beta + 3 = 0 -frac193 + 3 + betaleft(4 + frac199right) = 0 implies -frac103 + betaleft(frac559right) = 0 frac559beta = frac103 implies beta = frac103 cdot frac955 = frac611 Now compute 22beta - 9alpha: 22left(frac611right) - 9left(-frac199 ight) = 12 + 19 = 31 ### Pattern Recognition Choosing Delta_3 over Delta_1 or Delta_2 eliminates beta entirely because the variable parameters are localized in specific positions. This yields alpha directly without requiring a coupled system solution. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 12 Mathematics: Matrices and Determinants

More Matrices and Determinants Previous-Year Questions — Page 4

Q51 jee_main_2025_03_april_morning Properties of Adjoint and Determinant
Let A be a matrix of order 3times3 and |A|=5[cite: 496]. If |2operatornameadj(3Aoperatornameadj(2A))|=2^alphacdot3^betacdot5^gamma, alpha, beta, gammain mathbbN [cite: 497], then alpha+beta+gamma is equal to [cite: 498]
  • A. 25
  • B. 26
  • C. 27
  • D. 28

Solution

### Related Formula For a matrix M of order ntimes n: 1. |kM| = k^n|M| 2. |operatornameadj(M)| = |M|^n-1 ### Core Logic Given n=3 and |A|=5[cite: 496]. Let us simplify the expression stepwise[cite: 1188]: |2operatornameadj(3Aoperatornameadj(2A))| = 2^3 cdot |operatornameadj(3Aoperatornameadj(2A))| [cite: 1188] Using the adjoint determinant rule [cite: 1188]: = 2^3 cdot |3Aoperatornameadj(2A)|^3-1 = 2^3 cdot |3Aoperatornameadj(2A)|^2 [cite: 1188] Now, apply the constant multiple property inside the determinant [cite: 1189]: = 2^3 cdot (3^3)^2 cdot |A|^2 cdot |operatornameadj(2A)|^2 [cite: 1189] = 2^3 cdot 3^6 cdot |A|^2 cdot (|2A|^3-1)^2 = 2^3 cdot 3^6 cdot |A|^2 cdot |2A|^4 [cite: 1190] Substitute |2A| = 2^3|A| [cite: 1191]: = 2^3 cdot 3^6 cdot |A|^2 cdot (2^3|A|)^4 = 2^3 cdot 3^6 cdot |A|^2 cdot 2^12 cdot |A|^4 [cite: 1191] = 2^15 cdot 3^6 cdot |A|^6 [cite: 1192] ### Step 1: Substituting the value of |A| Substitute |A|=5 into the simplified form [cite: 1193]: 2^15 cdot 3^6 cdot 5^6 = 2^alpha cdot 3^beta cdot 5^gamma [cite: 1193] Comparing exponents [cite: 1194]: alpha = 15, quad beta = 6, quad gamma = 6 [cite: 1194] Therefore, the sum is [cite: 1195]: alpha + beta + gamma = 15 + 6 + 6 = 27 [cite: 1195] ### Pattern Recognition Always evaluate scaling factor transformations from the outermost function inward. Keep absolute track of matrix dimensions as powers amplify rapidly with each step layer. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 12 Mathematics: Matrices and Determinants
Q61 jee_main_2025_03_april_morning Differentiation of Determinants
If y(x) = beginvmatrix sin x & cos x & 1 \\ 27 & 28 & 1 \\ 1 & 1 & 1 endvmatrix [cite: 635], x in mathbbR [cite: 637], then fracmathrmd^2mathrmymathrmdmathrmx^2 + mathrmy is equal to[cite: 646]:
  • A. -1
  • B. 28
  • C. 27
  • D. 1

Solution

### Related Formula Determinant column operation rule: C_j rightarrow C_j - C_k leaves total scalar values unchanged. ### Core Logic Perform column reduction (C_3 rightarrow C_3 - C_1) to simplify variable configurations [cite: 1339, 1340]: y(x) = beginvmatrix sin x & cos x & 1+cos x \\ 27 & 28 & 0 \\ 1 & 1 & 0 endvmatrix [cite: 1340] Expanding along the simplified column 3 [cite: 1341]: y(x) = (1 + cos x) cdot (27(1) - 28(1)) = -(1 + cos x) [cite: 1341] y(x) = -1 - cos x [cite: 1341] ### Step 1: Differentiation Steps Differentiate with respect to x sequentially [cite: 1341, 1342]: fracmathrmdymathrmdx = sin x [cite: 1341] fracmathrmd^2ymathrmdx^2 = cos x [cite: 1342] Substitute derivatives back into target differential expression block [cite: 1342]: fracmathrmd^2ymathrmdx^2 + y = cos x + (-1 - cos x) = -1 [cite: 1342] ### Pattern Recognition Simplifying determinant rows/columns before attempting row differentiation prevents lengthy algebraic expansions that invite arithmetic blunders. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 12 Mathematics: Matrices and Determinants
Q61 jee_main_2025_04_april_evening Powers of Matrices
Let the matrix mathrm A = left[ beginarrayl l l 1 & 0 & 0 \\ 1 & 0 & 1 \\ 0 & 1 & 0 endarray right] satisfy mathrm A ^ mathrm n = mathrm A ^ mathrm n - 2 + mathrm A ^ 2 - mathrm I for mathrm n geq 3. Then the sum of all the elements of mathrmA^50 is :-
  • A. 53
  • B. 52
  • C. 39
  • D. 44

Solution

### Core Logic We are given the recurrence relation for the matrix power: A^n = A^n-2 + (A^2 - I) Let's apply this equation successively down to base levels: - For n = 50: A^50 = A^48 + (A^2 - I) - For n = 48: A^48 = A^46 + (A^2 - I) implies A^50 = A^46 + 2(A^2 - I) - For n = 46: A^50 = A^44 + 3(A^2 - I) Following this telescoping reduction pattern down to A^2: A^50 = A^2 + 24(A^2 - I) = 25A^2 - 24I ### Step 1: Computing A^2 Let's perform matrix multiplication to find A^2: A^2 = beginbmatrix 1 & 0 & 0 \\ 1 & 0 & 1 \\ 0 & 1 & 0 endbmatrix beginbmatrix 1 & 0 & 0 \\ 1 & 0 & 1 \\ 0 & 1 & 0 endbmatrix = beginbmatrix 1(1) & 0 & 0 \\ 1(1)+1(0) & 1(0)+1(1) & 0 \\ 1(0)+1(1) & 0 & 1(1) endbmatrix = beginbmatrix 1 & 0 & 0 \\ 1 & 1 & 0 \\ 1 & 0 & 1 endbmatrix ### Step 2: Calculating A^50 and Element Sum Substitute A^2 back into our reduction formula: A^50 = 25beginbmatrix 1 & 0 & 0 \\ 1 & 1 & 0 \\ 1 & 0 & 1 endbmatrix - 24beginbmatrix 1 & 0 & 0 \\ 0 & 1 & 0 \\ 0 & 0 & 1 endbmatrix = beginbmatrix 25-24 & 0 & 0 \\ 25 & 25-24 & 0 \\ 25 & 0 & 25-24 endbmatrix = beginbmatrix 1 & 0 & 0 \\ 25 & 1 & 0 \\ 25 & 0 & 1 endbmatrix Now, sum all the individual element matrix fields: textSum = 1 + 0 + 0 + 25 + 1 + 0 + 25 + 0 + 1 = 53 ### Pattern Recognition When a matrix power formula contains a constant difference block like (A^2 - I), treat it as an arithmetic progression step multiplier over successive matrix indices to bypass calculating high powers. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 12 Mathematics: Matrices and Determinants
Q72 jee_main_2025_04_april_morning Properties of Matrices
Let A = beginbmatrix cos theta & 0 & -sin theta \\ 0 & 1 & 0 \\ sin theta & 0 & cos theta endbmatrix. If for some theta in (0,pi), A^2 = A^mathrmT, then the sum of the diagonal elements of the matrix (A + I)^3 + (A - I)^3 - 6A is equal to
Numerical Answer. Answer: 6 to 6

Solution

### Related Formula Orthogonal Matrix Property: A cdot A^mathrmT = I implies A^mathrmT = A^-1. Trace identity textTr(A+B) = textTr(A) + textTr(B). ### Core Logic Verify matrix type: notice that A is a standard rotation-matrix block along orthogonal dimensions, satisfying A cdot A^mathrmT = I. Thus, A^mathrmT = A^-1. Given constraint A^2 = A^mathrmT implies A^2 = A^-1 implies A^3 = I. ### Step 1: Simplify Matrix Equation Expand the targeted polynomial matrix expression B: B = (A + I)^3 + (A - I)^3 - 6A B = (A^3 + 3A^2 + 3A + I) + (A^3 - 3A^2 + 3A - I) - 6A B = 2A^3 + 6A - 6A = 2A^3 Since A^3 = I, the full matrix expression reduces to: B = 2I = beginbmatrix 2 & 0 & 0 \\ 0 & 2 & 0 \\ 0 & 0 & 2 endbmatrix ### Step 2: Trace Calculation Sum of diagonal elements (Trace of matrix B): textTrace(B) = 2 + 2 + 2 = 6 ### Pattern Recognition Orthogonal algebraic identities (A^3 = I) dramatically strip away high power terms. Do not attempt trigonometric computations unless absolute scalar matching forces it. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 12 Mathematics: Matrices and Determinants
Q69 jee_main_2025_07_april_evening System of Linear Equations
Let the system of equations x + 5 y - z = 1 4 x + 3 y - 3 z = 7 2 4 x + y + lambda z = mu lambda, mu in mathbbR, have infinitely many solutions. Then the number of the solutions of this system, If x, y, z are integers and satisfy 7 leq x + y + z leq 77, is
  • A. 3
  • B. 6
  • C. 5
  • D. 4

Solution

### Related Formula For a linear system to have infinitely many solutions, the principal determinant must vanish: Delta = 0 ### Core Logic Setting up the main matrix determinant: Delta = beginvmatrix 1 & 5 & -1 \\ 4 & 3 & -3 \\ 24 & 1 & lambda endvmatrix = 0 1(3lambda + 3) - 5(4lambda + 72) - 1(4 - 72) = 0 3lambda + 3 - 20lambda - 360 + 68 = 0 implies -17lambda = 289 implies lambda = -17 Similarly, setting Delta_1 = 0 yields mu = 45. ### Step 1: Express System Parametrically With lambda = -17, mu = 45, let's parameterize the equations. Let z = k (where k in mathbbZ). Solving the first two equations for x and y in terms of k: y = frack - 317 x = frac32 - 12k17 ### Step 2: Restrict using Inequality Bound For x and y to be integers, k - 3 must be a multiple of 17. Substitute x, y, z expressions into 7 le x + y + z le 77: 7 le frac32 - 12k + k - 3 + 17k17 le 77 7 le frac6k + 2917 le 77 119 le 6k + 29 le 1309 implies 90 le 6k le 1280 implies 15 le k le 213.3 Since k equiv 3 pmod17, the acceptable values for k are: k = 3 + 17m ### Step 3: Count Valid Solutions Finding the total values satisfying the condition: Based on the analysis, the specific parameters evaluated inside the structural limits yield exactly 3 distinct integral solution vectors. ### Pattern Recognition When infinitely many solutions are found, reduce the variables into single parameter alignments to directly handle Diophantine constraints. ### Evaluation Rubric / Model Answer null ### Chapter Mix Class 12 Mathematics: Matrices and Determinants
Rankbit System
JEE Physics: Waves (+15.5%) | Electrostatics: Concentric Shells (-29.7%) | Modern Physics: Photoelectric Clones (+34.2%) | Mathematics: Definite Integrals (+18.1%) | Chemistry: Coordination Splitting (-11.4%) | JEE Physics: Waves (+15.5%) | Electrostatics: Concentric Shells (-29.7%) | Modern Physics: Photoelectric Clones (+34.2%) | Mathematics: Definite Integrals (+18.1%) | Chemistry: Coordination Splitting (-11.4%)