令a=sinx+cosxa²=sin²x+2sinxcosx+cos²x=1+2sinxcosx所以sinxcosx=(a²-1)/2所以f(a)=(a²-1)/2sin(π/2+π/6)=√3/2所以原式=f(√3/2)=[(√3/2)²-1]/2=-1/8