Question 743369
Please help me verify the identity: 
{{{(1-tan^2(t))/(1+tan^2(t))= 2cos^2(t)-1}}}
start with left side:
{{{(1-tan^2(t))/(1+tan^2(t))}}}
{{{(1-sin^2(t)/cos^2(t))/(1+sin^2(t)/cos^2(t))}}}
{{{(cos^2(t)-sin^2(t))/(cos^2(t)+sin^2(t))=(cos^2(t)-sin^2(t))/1=2cos^2(t)-1}}}
verified: left side=right side