第一题答案为cos(3x+y),因为cos（a+b）=cosacosb—sinasinb
第二题为cosy,因为原式等于cos（x-（x+y））=cos（-y）,cos值为正的象限是一和四,故cos（-y）又等于cosy
望采纳,谢谢!
化简cos2xcos(x+y)-sin2xsin(x+y)=
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