【解】Ccos(x-y)=√5/5且x sin(x-y)=-2√5/5cos2x=√10/10 ==> sin2x=3√10/10∵x+y=2x-(x-y)∴cos(x+y)=cos[2x-(x-y)]=cos2xcos(x-y)+sin2xsin(x-y)=√10/10×√5/5+3√10/10×(-2√5/5)=-√2/2∵x,y都是锐角∴(x+y...