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AP EAMCET · Maths · Continuity and Differentiability

Let \(f(x)\) be a differentiable function such that \(f(1)=2\), \(f(2)=6\) and \(f(x+y)=f(x)+k x y+\frac{4}{3} y^2 \forall x, y \in \mathbb{R}\) then \(f(x)=\)

  1. A \(4 x-2\)
  2. B \(y-4 x^2+2 x-4\)
  3. C \(\frac{8}{3} x^2+\frac{4}{3}\)
  4. D \(\frac{4}{3} x^2+\frac{2}{3}\)
Verified Solution

Answer & Solution

Correct Answer

(D) \(\frac{4}{3} x^2+\frac{2}{3}\)

Step-by-step Solution

Detailed explanation

\(f(1)=2, f(2)=6\) \(f(x+y)=f(x)+k x y+\frac{4}{3} y^2... (I)\) Put \(x=1 \& y=1\) \(\begin{aligned} & \therefore f(1+1)=f(1)+k+\frac{4}{3} \Rightarrow 6-\frac{4}{3}=2+k \\ & \Rightarrow k=\frac{8}{3} \end{aligned}\) Put \(x=1\) and \(y=m-1\) in equation (i), we get…