js在文本框中任意输入两个数，求它们之间的累加和

在文本框1和2中任意输入两个数字，例如3，10点击＝按钮，可以求出它们之间的累加和，把结果输入到第三个文本框中


<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <input id="a" value="" />
    <input id="b" value=""/>
    <input type="button" onclick="add()" value="=" />
    <input id="c" value=""/>
     <script>
        const a = document.getElementById('a')
        const b = document.getElementById('b')
        function add(){

            const c = Number(a.value) + Number(b.value)
            document.getElementById('c').value = c
        }
    </script>
</body>
</html>

写两个input
中间来个=按钮
获取两个input的值，a+b=c
返回C，然后innerHTML到第三个文本框

<body>
<input id="num1" type="text"/>
<input id="num2" type="text"/>
<button onclick="test()">=</button>
<input id="num3" type="text"/>
</body>
<script>
    const num1 = document.getElementById("num1");
    const num2 = document.getElementById("num2");
    const num3 = document.getElementById("num3");
    const test = () => {
        let len = Math.abs(num1.value - num2.value) + 1;
        num3.value = ((Number(num1.value) + Number(num2.value)) * len) / 2;
    };
</script>


<!DOCTYPE html>
<html lang="zh">
    <head>
        <meta charset="UTF-8">
        <meta name="viewport" content="width=device-width, initial-scale=1.0">
        <meta http-equiv="X-UA-Compatible" content="ie=edge">
        <title></title>
    </head>
    <body>
        <body>
            <input id="num1" type="text" />
            <input id="num2" type="text" />
            <button onclick="test()">=</button>
            <input id="num3" type="text" />
        </body>
        <script>
            const num1 = document.getElementById("num1");
            const num2 = document.getElementById("num2");
            const num3 = document.getElementById("num3");
            const test = () => {
                let len = Math.abs(num1.value - num2.value) + 1;
                num3.value = ((Number(num1.value) + Number(num2.value)) * len) / 2;
            };
        </script>
    </body>
</html>

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