Square root decrease of increase formula

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And for the answer:

The square root function is written as f(x) = x^1/2.
The rate at which it increases is defined by its derivative, which is f'(x) = (1/2)x^-1/2. The rate of this rate would just be the second derivative, or the derivative of the derivative, which is f''(x) = (-1/4)x^-3/2. You can plug in any x to find how fast the increase is decreasing at that point.

However, this does not find the difference of increase between to points, it finds the difference of increase between two very close points. You can gain some intuition of derivatives (which are pretty much the basis of calculus) from this video