JSFiddle - React, Tailwind, and code Playground

HTML

<body>
  <div ng-app="myapp" ng-controller="maytinh">
    <input type="text" ng-model="so1"><br>
    <input type="text" ng-model="so2"><br>
    <select ng-model="pheptoan">
      <option value="+">+</option>
      <option value="-">-</option>
      <option value="*">*</option>
      <option value="/">/</option>
    </select>
  <button ng-click="tinh()">xem ket qua</button>
  {{ketqua}}
  </div>
</body>

JavaScript

var myapp = angular.module("myapp", []);

myapp.controller("maytinh", function($scope){
$scope.tinh = function() {
  var so1 = parseInt($scope.so1);
  var so2 = parseInt($scope.so2);
  var pheptoan = $scope.pheptoan;

  if (pheptoan == '+') {
    return $scope.ketqua = 'ket qua la' + (so1 + so2);    
  }
  else if (pheptoan == '-') {
    return $scope.ketqua = 'ket qua la' + (so1 - so2);    
  }
  else if (pheptoan == '*') {
    return $scope.ketqua = 'ket qua la' + (so1 * so2);    
  }
  else {
    return $scope.ketqua = 'ket qua la' + (so1 / so2);    
  }
  };
});