Tangents and the circle inside

You will write functions for the circle inside a triangle: the three tangent lengths, the radius, and a test for a four-sided figure that fits round a circle.

Your tasks.

1. In tangent_lengths, return the tuple (from A, from B, from C) for a triangle with sides a = BC, b = CA and c = AB. Each length is s minus the opposite side, where s is half the perimeter.
2. In incircle_radius, return the radius of the incircle: the area divided by s, with the area from Heron's formula.
3. In circumscribes, return True when ab + cd equals bc + da. Then set ad to the missing side DA of a figure round a circle with AB = 7, BC = 10 and CD = 12.