13.
[tex]\it \ cos20^o=sin(90^o-20^o)=sin70^o>sin50^o[/tex]
14.
[tex]\it \left.\begin{aligned} \it sin3-sin1=2sin\dfrac{3-1}{2}cos\dfrac{3+1}{2}=2sin1cos2 \\ \\ \\ \it 2\in \Big(\dfrac{\pi}{2},\ \pi\Big) \Rightarrow cos2<0\ \ \ \ \ \ \ \ \ \end{aligned}\right\} \Rightarrow 2sin1cos2<0\Rightarrow\\ \\ \\ sin3-sin1<0 \Rightarrow sin3<sin1[/tex]
