Practicing Success
Let f be a non-negative function defined on the interval [0,1]. If $\int\limits_0^x \sqrt{1-\left\{f^{\prime}(t)\right\}^2} d t=\int\limits_0^x f(t) d t, 0 \leq x \leq 1$ and $f(0)=0$, then |
$f\left(\frac{1}{2}\right)<\frac{1}{2}$ and $f\left(\frac{1}{3}\right)>\frac{1}{3}$ $f\left(\frac{1}{2}\right)>\frac{1}{2}$ and $f\left(\frac{1}{3}\right)>\frac{1}{3}$ $f\left(\frac{1}{2}\right)<\frac{1}{2}$ and $f\left(\frac{1}{3}\right)<\frac{1}{3}$ $f\left(\frac{1}{2}\right)>\frac{1}{2}$ and $f\left(\frac{1}{3}\right)<\frac{1}{3}$ |
$f\left(\frac{1}{2}\right)<\frac{1}{2}$ and $f\left(\frac{1}{3}\right)<\frac{1}{3}$ |
We have, $\int\limits_0^x \sqrt{1-\left\{f'(t)\right\}^2} d t=\int\limits_0^x f(t) d t$ Differentiating w.r. to $x$, we get $\sqrt{1-\left\{f'(x)\right\}^2}=f(x)$ $\Rightarrow \left\{f'(x)\right\}^2=1-\{f(x)\}^2$ $\Rightarrow f'(x)= \pm \sqrt{1-\{f(x)\}^2}$ $\Rightarrow \frac{f'(x)}{\sqrt{1-\{f(x)\}^2}}= \pm 1$ $\Rightarrow \int\limits \frac{f'(x)}{\sqrt{1-\{f(x)\}^2}} d x= \pm \int\limits 1 . d$ $\Rightarrow \sin ^{-1}\{f(x)\}= \pm x+C$ It is given that $f(0)=0$. Therefore, $C=0$. ∴ $\sin ^{-1}\{f(x)\}= \pm x$ But, $f(x)>0$ for all $x \in[0,1]$. ∴ $\sin ^{-1}\{f(x)\}=x \Rightarrow f(x)=\sin x$ We know that $\sin x<x$ for all $x>0$ $\Rightarrow f(x)<x$ for all $x>0$ $\Rightarrow f\left(\frac{1}{2}\right)<\frac{1}{2}$ and $f\left(\frac{1}{3}\right)<\frac{1}{3}$ |