求证(1-tan^4A)cos^2A+tan^2A=1..

求证(1-tan^4A)cos^2A+tan^2A=1..
求证(1-tan^4A)cos^2A+tan^2A=1
..

求证(1-tan^4A)cos^2A+tan^2A=1..
求证(1-tan^4A)cos^2A+tan^2A=1
即证cos^2A-sin^2Atan^2A+tan^2A=1
即证-sin^2Atan^2A+tan^2A=sin^2A
即证-tan^2A+1/cos^2A=1
即证-sin^2A+1=cos^2A
即证cos^2A+sin^2A=1