y'\left( {x_1 \left( {x_0 } \right)} \right)x'_1 \left( {x_0 } \right) = \cos x_0 x'_1 \left( {x_0 } \right) = \cos x_0 - \frac{{d\cos x_0 \sin x_0 }}
{{\left( {1 + \cos ^2 x_0 } \right)^{\frac{3}
{2}} }}