Math和toFixed用法

```javascript
var x = 3;
var y = 1.1;
console.log(x + y);
console.log((x * y));
console.log(((x+y) * (x-y)));
var z = 9;
//开平方
console.log((Math.sqrt(z)));
//abs 绝对值
console.log((Math.abs(y/x)));

//固定精度
var x = 3;
var y =1.1;
var z = x * y;
// toFixed 设置精确小数点后几位
console.log(z.toFixed(2));
var x = 3;
var y = 1;
console.log((x+y).toFixed(10));