Question

Given 3 tanA tanB=1 show that 2cos(A+B)= cos(A-B)

Answer 1

so, tan a tanb =1/3given 3 tan a tanb=1
now  ;sina sinb /cos a cos b=1/3change them in sin and cosapply componendo and dividendo ruleuse the forlmulae for cos(a+b) and cos (a-b). you will get your answer.