抛物线方程

var x = data.curPos.left + 60,y = data.curPos.top + 20,
    x1= data.endPos.left,      y1 = data.endPos.top,
    x2= (x+x1)/2,               y2 = y1 - y;
//开始计算抛物线函数表达式
var a = ((x2 - x)*(y1 - y) - (x1 - x)*(y2 - y)) / ((x1*x1 - x*x)*(x2 - x) - (x2*x2 -x*x)*(x1 - x)),
    b = ((y1 - y) - a*(x1*x1 - x2*x2)) / (x1 -  x),
    c = y - a*x*x - b*x;