cos(x+y)=1/3,cos(x-y)=2/3.0＜x＜90º,60º＜y＜90º∴可得:0＜x＜30＜60＜y＜90ºsin(x+y)=(2√2)/3.cos(x+y)=1/3.sin(y-x)=(√5)/3,cos(y-x)=2/3.[[[1]]]cos2x=cos[(y+x)-(y-x)]=cos(x+y)cos(y-x...