(Cos x + sin x)^2 + (cos x - sin x)^2 = 2
alors:
Cos^2x + sin^2x + 2cosxsinx + cos^2x + sin^2x - 2cosxsinx =
(On sait que sin^2+cos^2=1)
alors = cos^2x+sin^2x + cos^2x + sin^2x = 1+1=2
1 + (tan x)^2 = 1/(cos x)^2
Alors:
(Tan x)^2 = (sinx/cosx)^2
1+ sinx^2/cosx^2 = cosx^2/cosx^2 + sinx^2/cosx^2
(On sait que sin^2+cos^2=1)
donc = 1/cosx^2
