Q114 / 155 · WBJEE 2023 · Mathematics

Let \(f(x)=x^m, m\) being a non-negative integer. The value of \(m\) so that the equality \(f^{\prime}(a+b)=f^{\prime}(a)+f^{\prime}(b)\) is valid for all \(a\), \(b > 0\) is

  • Correct answer
    A
    0
  • B
    1
  • Correct answer
    C
    2
  • D
    3

Solution

Hint : Let \(f(x)=x^m, m \geq 0, m \in I\) \(\begin{aligned} & f^{\prime}(a+b)=f^{\prime}(a)+f^{\prime}(b), a, b > 0 \\ & \Rightarrow m(a+b)^{m-1}=m a^{m-1}+m \cdot b^{m-1} \\ & (a+b)^{m-1}=a^{m-1}+b^{m-1} \quad f^{\prime}(x)=m n^{m-1} \\ & \text { for } m=0 \Rightarrow 0=0 \\ & (a+b)^{-1}=a^{-1}+b^{-1} \end{aligned}\) Not satisfied for \(m=1\) \(\begin{aligned} & (a+b)^0=a^{1-1}+b^{1-1} \\ & 1=2 \end{aligned}\) Not satisfied for \(m=2\) \(\begin{aligned} & (a+b)^{2-1}=a^{2-1}+b^{2-1} \\ & a+b=a+b \text { (satisfied) } \\ & \text { for } m=3 \\ & (a+b)^{3-1}=a^{3-1}+b^{3-1} \end{aligned}\) Not satisfied

Hence, the correct answer is options: 'A', 'C'.

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