Step-by-step explanation:
sin a = 4/5 then cos a = 3/5
sin b = 12/13 then cos b = -5/13 (because in quadrant 2)
then
cos (a+b) = cos a cos b - sin a sin b
cos (a+b) = 3/5 * (-5/13) - 4/5 * 12/13
cos (a+b) = -3/13 - 48/65
cos (a+b) = -15/65 - 48/65
cos (a+b) = - 63/65
cos (a+b) = -63/65
in decimal -0.969 ≈ -0,97
