js////案例 //输入斐波那契数列的项数num,求前num项的和

let num = prompt("请输入斐波那契数列的项数：");
let sum = 0;
let a = 0, b = 1;
for(let i=0; i<num; i++){
sum += b;
let temp = b;
b += a;
a = temp;
}
console.log(`前${num}项斐波那契数列的和为：${sum}`);