Need help on proving trig identitiy?

Question

prove that: tan(theta + alpha) = (tan(theta) + tan(alpha)) / (1 - tan(theta).tan(alpha))

Answer 1

The process is to take the sum formula for sin and divide it by the sum formula for cos.  Once you have the formulas on top of each other, then divide each of the four terms by cos(theta)cos(alpha).  This will convert the sin and cos into tangents.