Question 928604
Prove 1-tan^2 x/2 = 2cosx/ 1+ cosx
start with left side:
Identity: tan(x/2)=sinx/(1+cosx)
{{{1-tan^2(x/2)=1-((sinx)/(1+cosx))^2
=1-((sin^2(x))/(1+cosx)^2)
=1-((1-cosx)/(1+cosx))=(1+cosx-1+cosx)/(1+cosx)=2cosx/(1+cosx)}}}
verified: left side=right side