为书写简单,设sinA=x,cosA=y,有x^2+y^2=1
x-y=1/2,(x-y)^2=x^2+y^2-2xy=1-2xy=1/4,xy=3/8
x^3-y^3=(x-y)(x^2+xy+y^2)=1/2*(1+xy)=1/2*(1+3/8)=11/16
即差是11/16.