<script>

        var xb = prompt('请输入您的性别');

        // 判断性别

        if (xb == '男') {

            // 性别为男性

            var sg = Number(prompt('请输入您的身高(cm)'));

            var tz = Number(prompt('请输入您的体重(kg)'));

            var boy = (sg - 80) * 0.7;

            var boy_up = boy + 0.05 * boy;

            var boy_down = boy - 0.05 * boy;

            if (tz >= boy_down && tz <= boy_up) {

                alert('体重正常');

            } else if (tz < boy_down) {

                alert('太瘦了，要多补充营养哦');

            } else{

                alert('该锻炼身体减肥了哦')

            }

        } else if (xb == '女') {

            // 性别为女性

            var sg = Number(prompt('请输入您的身高(cm)'));

            var tz = Number(prompt('请输入您的体重(kg)'));

            var girl = (sg - 70) * 0.6;

            var girl_up = girl + 0.05 * girl;

            var girl_down = girl - 0.05 * girl;

            if (tz >= girl_down && tz <= girl_up) {

                alert('体重正常');

            } else if (tz < girl_down) {

                alert('太瘦了，要多补充营养哦');

            } else{

                alert('该锻炼身体减肥了哦')

            }

        } else {

            alert('请输入您的性别');

        }