The Chain Rule
Differentiate the outside, then multiply by the inside's derivative.
Concept
Rates multiply
If y depends on u and u depends on x, the rates compose: dy/dx = dy/du · du/dx.
In practice: treat the inner expression as a single blob, differentiate the outer function of that blob, then multiply by the blob's derivative.
d/dx f(g(x)) = f′(g(x)) · g′(x)