<div ng-controller="TestCtrl">
<input type="text" ng-model="w"/>
<input type="text" ng-model="h"/>
<input type="text" ng-model="s" />
<p>Площадь = {{s}}</p>
</div>
function TestCtrl($scope){
$scope.w = 2;
$scope.h = 3;
$scope.s = $scope.w+$scope.h;
}
$scope.$watch('[w,h]', function(values){
var w = values[0] || 0, h = values[1] || 0;
$scope.s = w*h;
}, true);