Chứng minh rằng: (tan2x - tanx)(sin2x - tanx) = tan²x; cotx - tanx = 2cot2x

Chứng minh rằng:
1. ( tan2x - tanx )(sin2x - tanx) = tan²x
2. Cotx - tanx = 2 cot2x
3. Cosx/(1 - sinx) = cot ( π/4 - x/2)