Substitution rule (change of variables) for Riemann integrals

Substitution rule: Let $f:[a,b]\to\mathbb{R}$ be continuous, and let $\varphi:[\alpha,\beta]\to[a,b]$ be continuously differentiable and monotone on $[\alpha,\beta]$. Then

(If $\varphi$ is decreasing, the right-hand side automatically changes sign because $\varphi(\alpha)>\varphi(\beta)$.)

This formula is the rigorous justification for substitution in calculus and is the one-dimensional prototype for higher-dimensional change of variables.