Find the angle between the two sides of length 6 in an isosceles triangle that has one side of length 9 and two sides of length 6. Find using the inverse.

In an isosceles triangle the altitude right-bisects the base, so we get a right-angled triangle

let the base angle be Ø
cosØ=4.5/6 = .75
Ø = 41.4°
so the angle between the 2 equal sides is 97.2°

or use the cosine law

9^2 = 6^2 + 6^2 - 2(6)(6)cosA
cosA = -9/72 = -.125
A = 97.2°