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 Post subject: Existence of $c$Posted: Sat Oct 29, 2016 10:52 am

Joined: Sat Nov 07, 2015 6:12 pm
Posts: 841
Location: Larisa
Let $f:[0, 1] \rightarrow \mathbb{R}$ be a continuous function such that $f(0)=0$ and

$$\int_0^1 x f(x) \, {\rm d}x = \int_0^1 f(x) \, {\rm d}x$$

Prove that there exists a $c \in (0, 1)$ such that $\displaystyle \int_0^c x f(x) \, {\rm d}x = \frac{c}{2} \int_0^c f(x) \, {\rm d}x$.

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 Post subject: Re: Existence of $c$Posted: Sun Jan 14, 2018 8:21 pm

Joined: Sat Nov 14, 2015 6:32 am
Posts: 159
Location: Melbourne, Australia
Let $I(x) = \bigintsss_{0}^{x} f(t) \, {\rm d}t$ and $G(x) = \bigintsss_{0}^{x} I(t) \, {\rm d}t$. Integrating by parts $(1)$ reveals that

$$\int_{0}^{1} I(t) \, {\rm d}t =0 \implies G(1) =0$$

Now let us consider the function $\displaystyle K(x) = \frac{G(x)}{x^2}$ . It holds that $K(1)=0$. As we can also see using two consecutive DeL’ Hospital’s Rules , it also holds that $\lim \limits_{x \rightarrow 0} K(x)=0$. So, by Rolle’s theorem there exists a $c \in (0, 1)$ such that

$$G\left ( c \right ) = \frac{c}{2} I(c)$$

However integration by parts reveals that

$$G(c) = c I(c) - \int_{0}^{c} x f(x) \, {\rm d}x$$

and thus $\displaystyle \int_{0}^{c} x f(x) \, {\rm d}x = \frac{c}{2} \int_{0}^{c} f(x) \, {\rm d}x$ which is the desired output.

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