Chứng minh các đẳng thức sau: a) $ \frac{sin^2a-cos^2a}{1+2sinacosa} = \frac{tana-1}{tana+1}$
b) $ \frac{sin^2a - tan^2a}{cos^2a - cot^2a} = tan^6a$
c) $ \frac{sin4a}{1+cos4a} . \frac{cos2a}{1+cos2a} = tana$
d) $ sin^4a + cos^4a = 1-2sin^2a.cos^2a = \frac{3}{4} + \frac{1}{4}cos4a $
e) $ \frac{cos^2a - sin^2a}{cot^2a - tan^2a} = sin^2a.cos^2a$