0 <= a <= pi/2, значит, sin a > 0, cos a > 0.
sin a = 12/13; sin^2 a = 144/169
cos^2 a = 1 - sin^2 a = 1 - 144/169 = (169 - 144)/169 = 25/169
cos a = 5/13
pi <= b <= 3pi/2, значит, sin b > 0, cos b < 0.
cos b = -0,6 = -3/5; cos^2 b = 0,36
sin^2 b = 1 - cos^2 b = 1 - 0,36 = 0,64
sin b = 0,8 = 4/5
sin (a - b) = sin a*cos b - cos a*sin b = 12/13*(-3/5) - 5/13*4/5 =
= -36/65 - 20/65 = -56/65