JEE Advanced · Mathematics · 27. Definite Integration
Let \(f: R \rightarrow R\) be a continuous function, which satisfies \(f(x)=\int_0^x f(t) d t\). Then, the value of \(f(\ln 5)\) is
- A 5
- B \(0\)
- C 1
- D 2
Answer & Solution
Correct Answer
(B) \(0\)
Step-by-step Solution
Detailed explanation
From given integral equation, \(f(0)=0\). Also, differentiating the given integral equation w.r.t. \(x\), we get
\[
\begin{gathered}
f^{\prime}(x)=f(x) \\
\text { If } \quad f(x) \neq 0 \Rightarrow \frac{f^{\prime}(x)}{f(x)}=1 \\
\Rightarrow \quad \log f(x)=x+C \Rightarrow f(x)=e^C e^x \\
\because f(0)=0 \Rightarrow e^C=0, \text { a contradiction } \\
\therefore f(x)=0, \forall x \in R \Rightarrow f(\ln 5)=0
\end{gathered}
\]
\[
\begin{gathered}
f^{\prime}(x)=f(x) \\
\text { If } \quad f(x) \neq 0 \Rightarrow \frac{f^{\prime}(x)}{f(x)}=1 \\
\Rightarrow \quad \log f(x)=x+C \Rightarrow f(x)=e^C e^x \\
\because f(0)=0 \Rightarrow e^C=0, \text { a contradiction } \\
\therefore f(x)=0, \forall x \in R \Rightarrow f(\ln 5)=0
\end{gathered}
\]
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