y=log2(1+cos（9/2π-a）)+log2(1+sin（a-5π）)
=log2[(1+cos(π/2-a))((1+sin(a-π)]
=log2[(1+sina)(1-sina)]
=log2(cos^2a)
=2log2|cosa|
a属于【0,π/4】,根号2/2=cosa=1
-1/2=log2cosa=0
-1=2log2|cosa|=0