A(x1,y1)B(x2,y2),c,(x3,y3)。AB:(y一y1)/(y1一y2)二(x一x1)/(x1一x2)。x1y一x2y一x1y1十x2y1二xy1一xy2一x1y1十x1y2,(y1一y2)x一(x1一x2)y十x1y2一x2y1二0。Bc:(y2一y3)x一(x2一x3)y十x2y3一x3y2二0。CA:(y3一y1)x一(x3一x1)y十x3y1一x1y3二0。设内心(a,b)。则(y1一y2)a十(x1一x2)b十x1y2一x2y1/√(x1一x2)^2十(y1一y2)^2,解方程组便求得