通過三個坐標(biāo)點充岛，計算角度
(function (){
let A = {X:0,Y:0}
let B = {X:4,Y:0}
let C = {X:4,Y:4}
var AB = Math.sqrt(Math.pow(A.X - B.X, 2) + Math.pow(A.Y - B.Y, 2));
var AC = Math.sqrt(Math.pow(A.X - C.X, 2) + Math.pow(A.Y - C.Y, 2));
var BC = Math.sqrt(Math.pow(B.X - C.X, 2) + Math.pow(B.Y - C.Y, 2));
var cosA = (
Math.pow(AB, 2) + Math.pow(AC, 2) - Math.pow(BC, 2)
) / (
2 * AB * AC
);
var angleA = Math.round( Math.acos(cosA) * 180 / Math.PI );
console.log(angleA)
// 得到angleA角度：45°
/*
AB = 開根( (A.X-B.X)2 + (A.Y-B.Y)2 ）
AC = A.X-C.X A.Y-C.Y
BC = B.X-C.X B.Y-C.Y
consA = (AB2+AC2-BC2) / (2*AB*AC)
angleA = Math.acos(cosA)*180/Math.PI
*/
})()
