sin(a/2)^4+cos(a/2)^4= [(sin(a/2))^2+(cos(a/2))^2]^2 - 2(sin(a/2))^2*(cos(a/2))^2
= 1 - 1/2 (sin(a))^2 利用 (sinx)^2+(cosx)^2=1 2sinxcosx=sin2x
=5/9
所以 (sina)^2=8/9
因为 a是第三象限的角,所以sina=-2√2/3