Steve M. answered 07/02/19
Algebra, Trig, Calculus -- Learn to Love it as I Do
Whenever you need to find some func(arcfunc(a/b)), draw the triangle which fits. In this case, let
sinA = 4/6
if you draw the triangle, you can see that tanA = 4/√20 = 2/√5
similarly, if cosB = 10/17, then tanB = √189/10 = 3√21/10
Now you just need to use the sum of tangents formula to find
tan(A+B) = (2/√5 + 3√21/10) / (1 - 2/√5 * 3√21/10) = -(289√5 + 135√21)/128
or maybe
(20√5 - 15√21) / (50 - 6√105)
or maybe just -9.8818