如何化简sin^2(-a-5π/2)-cos^2(a-7π/2)/{sin(a-3π/2)+cos(-a-3π/2)}

sin^2(-a-5π/2)-cos^2(a-7π/2)/{sin(a-3π/2)+cos(-a-3π/2)}=[-sin(2π+a+π/2)]^2-cos^2(-4π+a+π/2)/{sin(-2π+a-π/2)+cos(2π+a-π/2)}=[cos^2(a)-sin^2a]/{cosa+sina]=cosa-sina=√2sin(π/4-a)