Question 1064484
{{{tan(t/2)=0 +- sqrt((1-cos(t))/(1+cos(t)))}}}
{{{tan^2(t/2)=(1-cos(t))/(1+cos(t))}}}
So then(using a substitution {{{A=cos(t)}}} for better readability),
{{{t^2=(1-A)/(1+A)}}}
{{{(1+A)t^2=1-A}}}
{{{t^2+At^2=1-A}}}
{{{At^2+A=1-t^2}}}
{{{A(1+t^2)=1-t^2}}}
{{{A=(1-t^2)/(1+t^2)}}}
{{{highlight(cos(t)=(1-t^2)/(1+t^2))}}}