Question

How do you verify the identity: `sin(A+B) = (tanA+tanB)/(secA secB)`?

Answer 1

Right term, numerator:

`RN = sin A/cos A + sin B/cos B = (sin A.cos B + sin B.cos A)/(cos A.cos B)`

Right term denominator:

`RD = 1/cos A(1/cos B) = 1/(cos A.cos B)`

`(RN)/(RD) = (sin A.cos B + sin B.cos A) = sin (A + B)` Correct.