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JEE Mains · Maths · STD 12 - 1. relation and function

If \(g(x)=x^{2}+x-1\) and \((\operatorname{gof})(\mathrm{x})=4 \mathrm{x}^{2}-10 \mathrm{x}+5,\) then \(f\left(\frac{5}{4}\right)\) is equal to

  1. A \(\frac{3}{2}\)
  2. B \(-\frac{1}{2}\)
  3. C \(-\frac{3}{2}\)
  4. D \(\frac{1}{2}\)
Verified Solution

Answer & Solution

Correct Answer

(B) \(-\frac{1}{2}\)

Step-by-step Solution

Detailed explanation

\(g(x)=x^{2}+x-1\) \(g(f(x))=4 x^{2}-10 x+5\) \(=(2 x-2)^{2}+(2-2 x)-1\) \(=(2-2 x)^{2}+(2-2 x)-1\) \(\Rightarrow f(x)=2-2 x\) \(f\left(\frac{5}{4}\right)=\frac{-1}{2}\)
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