def evaluate_knn(knn, x_train, y_train, x_test, y_test):
    """
    Given a trained classifier, its training data, and test data, calculate
    the accuracy on the training and test sets.
    
    Input:    knn - A trained k-nn classifier
              x/y_train - Training data
              x/y_test  - Test data
    
    Returns:  A tuple (train_acc, test_acc) with the resulting accuracies,
              obtained when using the classifier on the given data.
    """
    train_score = knn.score(x_train, y_train)
    test_score = knn.score(x_test, y_test)
    return (train_score, test_score)