$\int\limits_0^\pi {xf\left( {\sin x} \right)dx} $ is equal to

Question

$\pi \int\limits_0^\pi {f\left( {\cos x} \right)dx} $

$\,\pi \int\limits_0^\pi {f\left( {sinx} \right)dx} $

${\pi \over 2}\int\limits_0^{\pi /2} {f\left( {sinx} \right)dx} $

$\pi \int\limits_0^{\pi /2} {f\left( {\cos x} \right)dx} $

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