Lagrange's identity (disambiguation)

Lagrange's identity may refer to:
 * Lagrange's identity, an algebraic identity
 * Lagrange's identity (boundary value problem), an identity in calculus
 * Lagrange's trigonometric identities, two trigonometric identities
 * Lagrange's four-square theorem, a theorem from number theory
 * Lagrange polynomial for theorems relating to numerical interpolation
 * Euler–Lagrange equation of variational mechanics