Q9 / 75 · NTA Abhyas (JEE Main) 2020 · Mathematics · Matrices

Consider the matrix A=x2yz2yzxzx2y and AAT=9I. If TrA>0 and xyz=16, then the value of x3+8y3+z3 is equal to (where, TrA, I and AT denote the trace of matrix A i.e. the sum of all the principal diagonal elements, the identity matrix of the same order of matrix A and the transpose of matrix A respectively)

  • A
    20
  • B
    22
  • C
    26
  • Correct answer
    D
    28

Solution

TrA>0⇒x+z+2y>0
AAT=x2yz2yzxzx2yx2yz2yzxzx2y
=x2+4y2+z22yx+2yz+xz2yx+2yz+zx2yx+2yz+xz4y2+x2+z22yz+2xy+zxzx+2xy+2yz2xy+2yz+zxx2+4y2+z2
∵AAT=9I⇒x2+4y2+z2=9, 2yx+2yz+xz=0
x+2y+z2=x2+4y2+z2+22xy+2yz+xz
x+2y+z2=9⇒x+2y+z=3 As, x+2y+z>0
x3+8y3+z3-6xyz=x+2y+zx2+4y2+z2-xz-2xy-2yz
⇒x3+8y3+z3-1=3×9
⇒x3+8y3+z3=28

Hence, the correct answer is option: 'D'.

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