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Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What is a derivative? Why are they important?

In Calculus with a teacher who hasn’t been able to explain the basic concept on a derivative. The internet has given me only complex answers and they haven’t helped me. Can you help?
gpt
In algebra you learned to calculate the slope of a straight line.  y = mx + b, right?  m is the slope.

But what about a curved line?  Then the slope isn't constant.  It changes: it's higher when the line is steeper, and lower when the line is flatter.  So instead of a single number, the slope is a function of x.  At any given x, your line has a certain y value, and a certain slope.

Calculus is the set of math tools that allow you to find the slope function from the original function, or the original function from the slope function.

The derivative is finding the slope function.  For example the derivative of y=x^2 is y'=2x.  That means a basic parabola has a slope of 2x.

Later you will learn about integrals, which are the opposite: they allow you to reverse the derivative, finding the original function from the slope function.