sin^x/2= cos^x/2= tan^x/2= cot^x/2=
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sin(x/2)^2=(1-cosx)/2
cos(x/2)^2=(1+cosx)/2
tan(x/2)^2=(1-cosx)/(1+cosx)=(1-cosx)^2/sinx^2=sinx^2/(1+cosx)^2
cot(x/2)^2=(1+cosx)/(1-cosx)=(1+cosx)^2/sinx^2=sinx^2/(1-cosx)^2
