Sin (a+b), если cosa=-4/5 cisb=5/13

Cosa =  √(1 - sin²a) = - √(1 - (4/5)²) = - √(9/25) = 3/5
sinb = - √(1 - cos²b) = √( 1 - (-5/13)²) = √(144/169) = - 12/13
sin(a - b) = sinacosb - cosasinb = 
= (- 4/5)*( 5/13) - (3/5)*(- 12/13) = - 4/13 + 36/65 = 16/65