cos(x-y)＝cosx＊cosy＋sinx＊siny
cosx+cosy=3/5①
sinx+siny=4/5②
由①＾2＋②＾2得：
2＋2（cosx＊cosy＋sinx＊siny）＝1
cos(x-y)＝cosx＊cosy＋sinx＊siny=-1/2