Q32 / 91 · KVPY 2018 (SB/SX) · Mathematics

Suppose A=abcd is a real matrix with non-zero entries, ad-bc=0 and A2=A. Then, a+d equals

  • Correct answer
    A
    1
  • B
    2
  • C
    3
  • D
    4

Solution

We have,

A=abcd

A2=abcdabcd

A2=a2+bcab+bdac+cdbc+d2

Given, A2=A and ad-bc=0

∴a2+bcab+bdac+cdbc+d2=abcd

ab+bd=b

ba+d=b

⇒a+d=1

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

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