已知三角形三边 a,b,c,求其内切圆半径 r;已知海伦公式:S = (p(p - a)(p - b)(p - c))^(1/2);已知:p = (a + b + c)/2,S = pr;得:r = S / p;所以:r = ((p-a)(p-b)(p-c)/p)^(1/2)