We may draw a vertical that bisects the vertex angle and the shorter side of the triangle, to form a right-triangle with the measurements:
hypotenuse = 8
length = 1.5
We may then apply:
sin(∅) = P / H
sin(∅) = 1.5 / 8
∅ = 10.8 ≈ 11°
This is half of the vertex angle, so the vertex angle is:
11 * 2 = 22°
