x^2 graph

The reason why we used delta = 0.009 rather than 0.01, is that if delta = 0.01, then not everything in the function falls into the green area, which is exactly what we are trying to prove happens [that is, for every part of the function that is in the blue area, it is also in the green area (the opposite isn't necessarily true)].

To see what I mean by the above statement, if you set D = 0.01, then you'll notice that the top part of the function is only inside of the blue part, which shouldn't happen.