cos(x+y)=4/5270°＜x+y＜360°所以sin(x+y)=-√[1-(4/5)^2]=-3/5cos(x-y)=-4/590°＜x-y＜180°所以sin(x-y)=√[1-(-4/5)^2]=3/5所以cos2x=cos[(x+y)+(x-y)]=cos(x+y)cos(x-y)-sin(x+y)sin(x-y)=(4/5)*(-4/5)-(-3/5...