Chứng minh rằng csc4A + csc8A = cot2A-cot8A?

# RHS = cot2A-cot8A #

# = (cos2A) / (sin2A) - (cos8A) / (sin8A) #

# = (cos2Asin8A-cos8Asin2A) / (sin2Asin8A) #

# = sin (8A-2A) / (sin2Asin8A) #

# = (2cos2Asin6A) / (2cos2Asin2Asin8A) #

# = (sin8A + sin4A) / (sin4Asin8A) #

# = (sin8A) / (sin4Asin8A) + (sin4A) / (sin4Asin8A) #

# = 1 / (sin4A) + 1 / (sin8A) #

# = csc4A + csc8A = LHS #