Question

If 3 Tan alpha tan beta = 1
Prove that 2cos(A+B) = cos(A-B)

Answer 1

given 3 tan a tanb=1

so, tan a tanb =1/3

change them in sin and cos

now sina sinb /cos a cos b=1/3

apply componendo and dividendo

use the forluae for cos(a+b) and cos (a-b). you will get your answer.