A triangle ABC has its inscribed circle drawn — the largest circle that fits inside, touching all three sides. Its centre is the incenter and its radius is r. You can drag the three corners. Joining the incenter to each corner splits the triangle into three smaller triangles, one on each side; each has that side as its base and the inradius r as its height. Adding their areas gives one half r times the perimeter, which is r times the semiperimeter s. So the triangle's area equals r times s. The inradius, the semiperimeter, and the area computed as r times s and directly are shown live and agree. An info button opens a drawer with the explanation.