Chain rule
The rate of change of a composition is the product of the local rates: dy/dx = dy/du · du/dx.
The chain rule says the overall rate is still the product of the local rates, each taken at the current point: dy/dx = dy/du · du/dx At x = 0.5, u = 2, so dy/du = 2u = 4 and du/dx = 2, giving dy/dx = 8.
See it in action
Try chain rule hands-on in the lesson The chain rule, part of the The maths behind AI course.