MCQ
Let $f$ be a continuous function defined on $[0,1]$ such that $\int_0^1 f^2(x) d x=\left(\int_0^1 f(x) d x\right)^2$. Then, the range of $f$
  • A
    has exactly two points
  • B
    has more than two points
  • C
    is the interval $[0,1]$
  • is a singleton

Answer

Correct option: D.
is a singleton
d
(d)

We have,

$\int \limits_0^1 f^2(x) d x=\left(\int  \limits_0^1 f(x) d x\right)^2, x \in[0,1]$

We know Cauchy Schwartz inequality

${\left[\int_a^b f(x) \cdot g(x) d x\right]^2 \leq \int_a^b(f(x))^2 d x }$

$\int_a^b(g(x))^2 d x$

Here, $g(x)=1$ and equality holds only when $\frac{f(x)}{g(x)}=\lambda$

So, $f(x)$ is constant function.

$\therefore f(x)$ is a singleton.

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