def make_derivative(f):
    """Returns a function that approximates the derivative of f.

    Recall that f'(a) = (f(a + h) - f(a)) / h as h approaches 0. We will
    approximate the derivative by choosing a very small value for h.

    >>> square = lambda x: x*x
    >>> derivative = make_derivative(square)
    >>> result = derivative(3)
    >>> round(result, 3) # approximately 2*3
    6.0
    """
    h=0.00001
    def derivative(x):
        return (f(x+h)-f(x))/h
    return derivative