第一种方法：

    <div ng-app="">
        <div>
            <!--文本-->
            <input type="number" ng-model="A" placeholder="  输入数字1">
            {{a}}
            <input type="number" ng-model="B" placeholder="  输入数字2">
            =
            <!--//四个答案-->
            <h2 id="a1" style="display:none" onclick="w1()">{{A+B}}</h2>
            <h2 id="a2" style="display:none" onclick="w2()">{{A-B}}</h2>
            <h2 id="a3" style="display:none" onclick="w3()">{{A*B}}</h2>
            <h2 id="a4" style="display:none" onclick="w4()">{{A/B}}</h2>
        </div>
        <div>
            <!--// 四个按钮 加减乘除 点击哪一个显示哪一个-->
            <button id="AA" ng-click="a='+'">+</button>
            <button id="BB" ng-click="a='-'">-</button>
            <button id="CC" ng-click="a='*'">*</button>
            <button id="DD" ng-click="a='/'">/</button>
        </div>

        <script type="text/javascript">
            // <!-- 点击显示的，隐藏其它的-->
            var AA = document.getElementById("AA");
            var q = document.getElementById("a1");
            var BB = document.getElementById("BB");
            var w = document.getElementById("a2");
            var CC = document.getElementById("CC");
            var e = document.getElementById("a3");
            var DD = document.getElementById("DD");
            var r = document.getElementById("a4");

            AA.onclick=function(){
                q.style.display = "block";
                w.style.display = "none";
                e.style.display = "none";
                r.style.display = "none";
            }
            BB.onclick=function(){
                q.style.display = "none";
                w.style.display = "block";
                e.style.display = "none";
                r.style.display = "none";
            }
            CC.onclick=function(){
                q.style.display = "none";
                w.style.display = "none";
                e.style.display = "block";
                r.style.display = "none";
            }
            DD.onclick=function(){
                q.style.display = "none";
                w.style.display = "none";
                e.style.display = "none";
                r.style.display = "block";
            }
        </script>
     </div>

第二种方法：

    <div ng-app="" >
        <div>
            <button ng-click="a='+';C = A+B">+</button>
            <button ng-click="a='-';C = A-B">-</button>
            <button ng-click="a='*';C = A*B">*</button>
            <button ng-click="a='/';C = A/B">/</button>
        </div>
        <div>
            <input type="number" ng-model="A" placeholder="  输入数字1">
            {{a}}
            <input type="number" ng-model="B" placeholder="  输入数字2">
            =
            <input type="number" value="{{C}}" placeholder="  输入数字2">
        </div>
    </div>