高一的两角和余差的正余玹sin105sin60-cos105cos1201.cos60是多少2.sin105sin60-cos105cos120是多少3.cos〔x+g〕=0.8cos〔x-g〕=0.8cosxcosg是多少4cos〔15-x〕=cos15-sin〔165+x〕sin〔-15〕=?

Answer 1

1.cos60=1/2（这..公理）2.因为cos120=-cos60,所以原式=cos(105-60)=cos(45)=根号2/2(两角差的余弦公式）3.因为cos(x+g)=cosxcosg-sinxsing=0.8,cos(x-g)=cosxcosg+sinxsing=0.8,所以cosxcosg=0.8（两式相加除2）4....