Question

Establish the identity
cos(10theta) - cos(4theta)/cos(10theta) + cos(4theta) = -tan(7theta)tan(3theta)

Answer 1

cos c+ cos d = 2 cos (c+d)/2 cos (c-d)/2
and
cos c- cos d = - 2 sin (c+d)/2 sin (c-d)/2

Answer 2

here c is 10 theta
and
d is 4 theta