Question

If tan A=3/4,A>90 and tan B=5/12,B<90 evaluate cos(A+B)

Answer 1

Answer:

sinA = 3/5 ,sinB=5/13 ,cosA =4/5 ,cosB=12/13

cos(A+B) = cosAcosB-sinAsinB

cos(A+B) = 4/5*12/13-3/5*5/13 = 48/65-15/65=33/65