How prove this $\left(\int_{-\pi}^{\pi}f(x)\sin{x}dx\right)^2+\left(\int_{-\pi}^{\pi}f(x)\cos{x}dx\right)^2\le\frac{\pi}{2}\int_{-\pi}^{+\pi}f^2(x)dx$ Prove or disprove:

if $f(x)\ge 0,\forall x\in [-\pi,\pi]$,show that
$$\left(\int_{-\pi}^{\pi}f(x)\sin{x}dx\right)^2+\left(\int_{-\pi}^{\pi}f(x)\cos{x}dx\right)^2\le\dfrac{\pi}{2}\int_{-\pi}^{+\pi}f^2(x)dx$$

I can prove this if $2\pi$ takes the place of $\dfrac{\pi}{2}$

because  use Cauchy-schwarz inequality  we have
$$\left(\int_{-\pi}^{\pi}f(x)\sin{x}dx\right)^2\le\int_{-\pi}^{\pi}\sin^2{x}dx\int_{-\pi}^{\pi}f^2(x)dx$$

$$\left(\int_{-\pi}^{\pi}f(x)\cos{x}dx\right)^2\le\int_{-\pi}^{\pi}\cos^2{x}dx\int_{-\pi}^{\pi}f^2(x)dx$$
add this two  inequality, we have
$$\begin{align*}\left(\int_{-\pi}^{\pi}f(x)\sin{x}dx\right)^2+\left(\int_{-\pi}^{\pi}f(x)\cos{x}dx\right)^2 &amp;\le \int_{-\pi}^{\pi}f^2(x)dx\int_{-\pi}^{\pi}(\sin^2{x}+\cos^2{x})dx\\
&amp;=2\pi\int_{-\pi}^{+\pi}f^2(x)dx\end{align*}$$

see this Discrete form of inequality：Prove this inequality with Cauchy-Schwarz inequality

So far, I haven't found any counterexamples，such  $f(x)=1,\sin{x}+1$ it such this inequality
Note that 
\begin{align*}
\int_{-\pi}^\pi f(x) \sin x \,dx 
&amp;= \int_0^{\pi/2} (f(x) - f(x - \pi) + f(\pi - x) - f(-x)) \sin x \,dx \\
&amp;= \int_0^{\pi/2} (g(x) + h(x)) \sin x \,dx
\end{align*}
and similarly
\begin{align*}
\int_{-\pi}^\pi f(x) \cos x \,dx 
&amp;= \int_0^{\pi/2} (f(x) - f(x - \pi) - f(\pi - x) + f(-x)) \cos x \,dx \\
&amp;= \int_0^{\pi/2} (g(x) - h(x)) \cos x \,dx
\end{align*}
where we define $g, h : [0, \pi/2] \to \mathbb{R}$ by $g(x) = f(x) - f(x - \pi)$ and $h(x) = f(\pi - x) - f(-x)$.

Then by Cauchy-Schwarz, 
\begin{align*}
\left(\int_0^{\pi/2} (g(x) + h(x)) \sin x \,dx\right)^2 
&amp;\leq \int_0^{\pi/2} \sin^2 x \,dx \int_0^{\pi/2} (g(x) + h(x))^2 \,dx \\
&amp;= \frac{\pi}{4} \int_0^{\pi/2} (g(x) + h(x))^2 \,dx
\end{align*}
and 
\begin{align*}
\left(\int_0^{\pi/2} (g(x) - h(x)) \cos x \,dx\right)^2 
&amp;\leq \int_0^{\pi/2} \cos^2 x \,dx \int_0^{\pi/2} (g(x) - h(x))^2 \,dx \\
&amp;= \frac{\pi}{4} \int_0^{\pi/2} (g(x) - h(x))^2 \,dx
\end{align*}
hence 
\begin{align*}
\left(\int_{-\pi}^\pi f(x) \sin x \,dx\right)^2 &amp;+ \left(\int_{-\pi}^\pi f(x) \cos x \,dx\right)^2 \\
&amp;\leq \frac{\pi}{4} \int_0^{\pi/2} (g(x) + h(x))^2 \,dx + \frac{\pi}{4} \int_0^{\pi/2} (g(x) - h(x))^2 \,dx\\
&amp;= \frac{\pi}{2} \int_0^{\pi/2} (g(x))^2 + (h(x))^2 \,dx \\
&amp;\leq \frac{\pi}{2} \int_0^{\pi/2} (f(x))^2 + (f(x - \pi))^2 + (f(\pi - x))^2 + (f(-x))^2 \,dx \\
&amp;= \frac{\pi}{2} \int_{-\pi}^\pi (f(x))^2 \,dx
\end{align*}
as desired, where the last inequality holds because $f$ is nonnegative.
This is a consequence of parseval identity. Let $\mathcal{H}$ a Hilbert space (dont worry all you need to know is that you have an inner product and the concept of orthogonal vectors, i.e. when $\langle f, g\rangle = 0$ and that the norm is given by $\langle f, f \rangle = \lVert f \rVert^2$) and let $\{e_n\}$ be a numerable orthonormal set then if $f \in \mathcal{H}$
$$
\sum_n \lvert \langle e_n, f\rangle \rvert^2 \leq \lVert f \rVert^2.
$$
with equality if $\{e_n\}$ is a basis. In this case
$$
\lVert f \rVert^2 = \int_{-\pi}^{\pi} \lvert f(x) \rvert^2 \,dx
$$
and
$$
e_1(x) = A\cos(x), \quad e_2(x) = A\sin(x)
$$
where $A$ is chosen so they are normal. 

Parseval identity is deduced from the fact that for orthonormal (it also works with orthogonal, but the last equality is not true) vectors $u_n$ one has
$$
\lVert \sum a_n u_n \rVert^2 = \sum \lVert  a_n u_n \rVert^2 = \sum \lvert  a_n \rvert^2
$$
and you can write
$$
f = \langle e_1,f \rangle e_1 + \langle e_2, f\rangle e_2 + (f - \langle e_1,f \rangle e_1 + \langle e_2, f\rangle e_2)
$$
where the 3 parts of the sum are orthogonal and you have that the norm of $f$ is bigger than the norm of the first 2 terms.

EDIT: it seems the constants are off. It does not coincide with the one in parseval's.