1+tan^2x=1+sin^2x/cos^2=cos^2x/cos^2x+sin^2/cos^2x=(cos^2x+sin^2x)/cos^21/1+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...