Question

tanx=sinx/cosx cos^2=1/1+tan 推倒

Answer 1

1+tan^2x

=1+sin^2x/cos^2

=cos^2x/cos^2x+sin^2/cos^2x

=(cos^2x+sin^2x)/cos^2

1/1+tan^2x

=1/[(cos^2x+sin^2x)/cos^2x]

=cos^2x/cos^2x+sin^2x

因为cos^2x+sin^2x=1

所以cos^2x/cos^2x+sin^2x=cos^2

即cos^2=1/1+tan^2