If $\int_x^{x+T}f(t)dt=C$ then $f$ is periodic

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If $f$ is a continious function from $\mathbb{R}$ to $\mathbb{R}$ and there exists $C$ such that:

$\forall x\in\mathbb{R} :\int_x^{x+T}f(t)dt=C$

then $f$ is $T$-periodic.

Proving the inverse is easier and has already been answer on another question.

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HINT:

Differentiate both sides of the equation $F(x)=\int_x^{x+T}f(t)\,dt=C$ with respect to $x$. What can you immediately conclude?