sin^(x/2)=(1-cos x)/2
sin(x/2)=25/26
sin x/2 = 正负5/root 26
因为x在(pie,3pie/2)
x/2在（pie/2,3pie/4)
sin x/2 >0
sin x/2 =5/root 26
cos x/2=(1-sin x/2）开方
取负值
tan x/2=(sin x/2)/(cos x/2)