Triangle Maths

Drag P, Q and R anywhere. The outer circle is the circumcircle, the one circle through all three corners, and it is worked out from wherever you put them; its centre is O. Put the three points in a straight line and there is no such circle, so none is drawn.

Tick Keep P, Q, R on one circle to pin them to a single circle and slide them around it, the way the original app worked.

The yellow circle inside is the incircle: the largest circle that fits in the triangle. Its centre I is where the three angle bisectors meet, and it touches each side at J, K and M — the feet of the perpendiculars from I.

Coordinates are measured from the origin where the axes cross, with x to the right and y upwards.