cos(x+75)=1/3
sin(x-15)=-sin(15-x)=-sin[90-(x+75)]=-cos(x+75)=-1/3
x在第三象限
cos(x+75)>0,所以x+75在第四象限
所以sin(x+75)<0
[cos(x+75)]^2+[sin(x+75)]^2=1
[sin(x+75)]^2=8/9
sin(x+75)=-2√2/3
cos(15-x)=cos[90-(x+75)]=sin(x+75)=-2√2/3
所以cos(15-x)+sin(x-15)
=-(2√2+1)/3