> function f1(r) {
> var l= Math.PI*r*r;
> return alert(l); }
> console.log(f1(prompt("输入圆的半径")));
> function f1(r) {
> var l= Math.PI*r*r;
> return alert(l); }
> console.log(f1(prompt("输入圆的半径")));