因为a属于(π/2,3π/4),
所以cosa0
根号1/2+1/2根号1/2+1/2sin2a
=根号1/2+1/2根号1/2+1/2(2cos^2a-1)
=根号1/2-1/2cosa
=根号1/2-1/2(1-2sin^2(a/2))
=根号sin^2(a/2)
=sin(a/2)