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//利用3种循环来计算1+2+3+…+100的值。

/*while循环*/
var a = 1;
var sum = 0;
//如果n小于等于100，则会执行while循环
    while (a <= 100)
    {
        sum=sum+a;
        a=a+1;
    }
document.write("while循环结构："+"1+2+3+…+100 = " + sum);

document.write('<br/>-----------------------------<br/>');

//do...while循环结构
var b = 1;
var sum1 = 0;
do
    {
        sum1 += b;
        b++;
    }
    while (b <= 100);
document.write("do...while循环结构："+"1+2+3+…+100 = " + sum1);

document.write('<br/>-----------------------------<br/>');

//for循环结构
var sum3=0;
for(var i=1;i<=100;i++){
    sum3=sum3+i;
}
document.write("for循环结构："+"1+2+3+…+100 = "+sum3);