死算出来的。一直没想出纯几何方法求h

h = hight of the arc. and A = 2*Pi*R*h

L = tangent line. 2a = chord.

L^2 = r*(2R + r), a^2 = h*(2R - h), and substitue them in

(h + r)^2 = L^2 - a^2

h = Rr/(2(R+r)).