已知：0<x<π/2<y<π,且sin(x+y)=5/13.(1)若tan(x/2)=1/2,分别求出cosx及cosy的值；(2)试比较siny与sin(x+y)的大小，并说明理由.
參考答案:cosx={1-[tan(x/2)]平方}/{1+[tan(x/2)]平方}=3/5
sinx=4/5
cos(x+y)=-12/13
cosy=cos(x+y-x)=cos(x+y)*cosx+sin(x+y)*sinx=-16/65
siny=63/65>sin(x+y)
(π/2<x+y<3π/2)