Practicing Success
Let f : R → R be a function satisfying $f(x+y)=f(x)+\lambda x y+3 x^2 y^2$ for all $x, y \in R$. If f(3) = 4 and f(5) = 52, then f'(x) is equal to |
$10 x$ $-10 x$ $20 x$ $128 x$ |
$-10 x$ |
We have, $f(x+y)=f(x)+\lambda x y+3 x^2 y^2$ for all $x, y \in R$ Putting x = 3 and y = 2, we get $f(5)=f(3)+6 \lambda+108$ $\Rightarrow 52=4+6 \lambda+108$ $\Rightarrow \lambda=-10$ ∴ $f(x+y)=f(x)-10 x y+3 x^2 y^2$ $\Rightarrow \frac{f(x+y)-f(x)}{y}=-10 x+3 x^2 y$ $\Rightarrow \lim\limits_{y \rightarrow 0} \frac{f(x+y)-f(x)}{y}=\lim\limits_{y \rightarrow 0}-10 x+3 x^2 y$ $\Rightarrow f'(x)=-10 x$ |