HTML：

 <div class="outer">
        <div class="left">我在左邊</div>        
        <div class="right">我在右邊</div>   

    </div>
    <div class="footer">我在底部</div>

方法一：

body{
    text-align: center;
    .outer{
        :after{
            display: block;
 

            content:'';
            clear: both;
            visibility: hidden;

        }
        .left{
            float: left;
            width:500px;
            height: 500px;
            line-height:500px;
            background-color: darkcyan;
        }
        .right{
            width:100%;
           // margin-left:500px;
 

           // height: 100px;
           line-height: 500px;
            background-color: gold
        }

    }
    .footer{
        background-color: green;
        height: 100px;
        line-height: 100px;
    }
}

方法二：

body{
    text-align: center;
    .outer{
        position: relative;
        .left
 
{
            position: absolute;
            left:0;
            top: 0;
            width: 500px;
            height: 500px;
            line-height: 500px;
            background-color: firebrick;
        }
        .right{
             //width:100%;
            height: 500px;
            line-height: 500px;
            margin-left:500px;
            background-color: fuchsia;
        }
    }
    .footer{
        background-color: gold;
        height:100px;
        line-height: 100px;
    }
}

方法三：

body{
    text-align:center;
    .outer{
        display: table;
        width: 100%;
    }
    .left,.right{
        display:table-cell;
        line-height: 500px;
    }
    .left{
        width:200px;
        height: 500px;
        background-color: deepskyblue;
    }
    .right{
        background-color: darkorchid;
    }
    .footer{
        height: 100px;
        line-height: 100px;
        background-color: darkkhaki;
    }
}

方法四：

body{
    text-align: center;
    .outer{
        display: flex;
        width:100%;
        line-height: 200px;
        .left{
            flex:0 0 200px;
            height: 200px;
            background-color: chartreuse;
        }
        .right{
            width: 100%;
            background-color: blueviolet;
        }
    }
    .footer{
    height: 100px;
    line-height: 100px;
    background-color: darkgray;
}
}