cosy=cos[(x+y)-x]
=cos(x+y)cosx + sin(x+y)sinx
x y为锐角,所以x+y小于180°,所以sin(x+y)>0
sin(x+y)=√(1-(-3/5)²)=4/5
sinx=√(1-(2/3)²)=√5/3
cosy=cos(x+y)cosx + sin(x+y)sinx
= -2/5 + 4√5 / 15