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Differential Equations

A differential equation relates a quantity to its own rate of change — which is how every law of physics is stated. The solver names the method (separable, integrating factor, characteristic equation), solves, verifies by substitution, and draws the family of solutions.

_Öyrən

Core Separable equations dy/dx = f(x)g(y): separate, integrate both sides, solve for y. y' = 2y Core First-order linear equations y′ + P(x)y = Q(x) and the integrating factor that makes it integrable. y' + y = x Core Second-order, constant coefficients ay″ + by′ + cy = 0 and the characteristic equation: oscillation, damping, growth. y'' + y = 0 Advanced Nonhomogeneous equations Driving forces: homogeneous solution plus a particular solution. y'' - 3y' + 2y = e^x Core Modelling with differential equations Growth, decay, cooling, mixing: writing the equation from the sentence. y' = 0.05*y

Symbols used here

e
Euler's number
2.71828…, the base whose exponential is its own derivative.
\lim_{x \to a} f(x)
limit
The value f(x) approaches as x approaches a.
f'(x),\ \frac{dy}{dx}
derivative
Instantaneous rate of change; slope of the graph.
\int f(x)\,dx,\ \int_a^b
integral
Antiderivative (indefinite) or signed area from a to b (definite).
y',\ y''
first and second derivative of y
Prime notation for derivatives with respect to x (or t).
C,\ C_1,\ C_2
arbitrary constants
Constants of integration fixed by initial conditions.
\frac{\partial u}{\partial t},\ \nabla^2 u
partial derivative in time, Laplacian
Rate of change in time; sum of second partials (the diffusion operator).

Questions people ask

What is a differential equation?

An equation whose unknown is a function, relating it to its own derivatives. "The rate of growth is proportional to the population" is y′ = ky, and solving it means finding y as a function of time.

Why does the solution have arbitrary constants?

Integrating loses information: many functions share the same derivative. An n-th order equation has n constants, fixed by n initial or boundary conditions.

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