Q55 / 125 · BITSAT 2012 · Mathematics

Let $f(x+y)=f(x) \cdot f(y)$ for all $x, y$ where $f(0) \neq 0$ If $f(5)=2$ and $f^{\prime}(0)=3,$ then $f^{\prime}(5)$ is equal to-

  • Correct answer
    A
    6
  • B
    0
  • C
    1
  • D
    None of these

Solution

$f^{\prime}(5)=\lim _{h \rightarrow 0} \frac{f(5+h)-f(5)}{h}$ $=\lim _{h \rightarrow 0} \frac{f(5+h)-f(5+0)}{h}$ $=\lim _{h \rightarrow 0} \frac{f(5) \cdot f(h)-f(5)+f(0)}{h}$ $(\because f(x+y)=f(x), f(y)$ for all $x, y)$ $=\left(\lim _{h \rightarrow 0} \frac{f(h)-f(0)}{h}\right) \cdot f(5)=f^{\prime}(0) \cdot f(5)$ $=3 \times 2=6$

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

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