A and b are the angles of the second quadrant, then

Cosa=- 12/ 13 from Sina =5/ 13.

Sinb=4/5 from cosb=-3/5.

So sin (a-b) = 5/13 × (-3/5)-(-12/13) × (4/5) = 33/65.

cos(a-b)= =(- 12/ 13)×(-3/5)+5/ 13×4/5 = 56/65

tan(a-b)=33/65÷ 56/65=33/56

From sin (a-b) >: 0, we know that a-b is the first or second quadrant angle.

From cos (a-b)>0, a-b is the first or fourth quadrant angle.

So a-b is the first quadrant angle.

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