(1 + cos2a) = tan²x

Chứng minh rằng:
1. ( sin2a)/(1+cos2a) = tana
2. (1- cos2a)/(1+ Cos2a) = tan²x
3. Cotx - cot2x = 1/(sin2x)
4. (1 + sin2x)/(cos2x) = cot [ (π/4) - x)]