Just want to check if there's an existing function for this before I code it

Say there’s a tri made of three vertices, A-B-C. I want to offset B by N mm from it’s current position on the X,Y plane. I want this translation to take place such that A-(Old B)-(New B) has the same angle as C-(Old B)-(New B). Or to put it another way, so that if A-B-C were a slice of cake, Old B-New B would cut it in half.

This isn’t especially difficult to code, just time consuming, and I figure anything I write will probably be less efficient than what’s already written. I’ve had a look through blender’s geometry mathutils, but I don’t understand the names of a lot of them. If anyone knows any of them which do some or all of this then I’d be grateful for having it pointed out.

For anyone else who ever comes across this, I don’t think there is an existing function, but the simplest way to code it is to find the incenter of the tri and use that to get the angle.