向量a=(cosx,sinx),向量b=(cosy,siny),
则a^2=1,b^2=1,
ab= cosx cosy+sinx siny=cos(x-y),
|a-b|=2√5/5,
平方得：|a-b|^2=4/5,
即a^2+b^2-2ab=4/5,
1+1-2 cos(x-y) =4/5,
cos(x-y) =3/5.
－π／2＜y＜0,且siny＝－5／13,
则cosy=12/13.
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