Question

1-tan^2x/1+tan^2x=2 cos^2x-1

Answer 1

(1 - sin^2A/cos^2A)/1+sin^2/cos^2

{(cos^2A - sin^2A)/cos^2A}/cos^2+sin^2/cos^2

(cos^2 -sin^2)/1

cos^2 -sin^2

cos^2 + cos^2 -1

2cos^2 -1

hence proved