Symbol
Problem
$0.$ $f:R→R,$ $f\left(x\right)=x^{2}$ $,-\pi 3x\leq \pi$ and $f\left(x+2\pi \right)=f\left(x\right),y_{x∈\right)}$ eR. If the fourier series $\infty$ of $\left(x\right)$ is represented as $f\left(x\right)=\sum a_{n}$ $cos$ nx, $n=0$ then $a_{0}=$ $1\right)$ $\dfrac {2\pi ^{2}} {3}$ $2\right)$ $\dfrac {\pi ^{2}} {3}$ $3\right)$ $\dfrac {4\pi ^{2}} {3}$ $4\right)$ $\dfrac {5\pi ^{2}} {3}$