x对应角为a，y对应角为b
则z=xcosa+ycosb
x^2+y^2=(xcosa+ycosb)^2
x^2(sina)^2+y^2(sinb)^2=2xcosa*ycosb
(xsina-ysinb)^2=2xycosa*cosb-2xycosa*cosb=2xycos(a+b)
因此2xycos(a+b)=0，cos(a+b)=0，0<a+b<180度，a+b=90度
则z=xcosa+ycosb
x^2+y^2=(xcosa+ycosb)^2
x^2(sina)^2+y^2(sinb)^2=2xcosa*ycosb
(xsina-ysinb)^2=2xycosa*cosb-2xycosa*cosb=2xycos(a+b)
因此2xycos(a+b)=0，cos(a+b)=0，0<a+b<180度，a+b=90度
