>>14114268It's called the chain rule. You have:
(complex function)^2
You first take that derivative
1/2 * (complex function)
and then you take the derivative of complex function, and multiple it.
Unless you want to get into fundamental proofs, accept it at face value. It's the associative property, multiply things. And calculus depends on the mean value theorem. If two points are going to be considered independent, the line connecting them on a function always have a slope the same as the shortest path between them no matter what the function does, as long as it doesn't break continuitiy.
The thing you should be focusing on is the relationship between trig functions. Here's a hint, plug in 0 or 1. Quick smoke test if what you tried makes sense.
