By the law of cosines in triangle formula_15,:formula_16Since the circles formula_17 tangent to each other::formula_18Let formula_19 be a point on the circle formula_1.
По теореме косинусов в треугольнике formula_15,: formula_16Поскольку окружности formula_17 касаются,: formula_18Пусть formula_19 — точка на окружности formula_1.