> ## Documentation Index
> Fetch the complete documentation index at: https://docs.truemath.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# Activities

> Activities are the equation units of a domain. Each activity relates a set of variables and can solve for different variables depending on which values are known.

An **activity** is an equation unit within a [domain](/concepts/domains). It defines a
relationship among [variables](/concepts/variables) — for example, the relationship
between a loan amount, an interest rate, a term, and a monthly payment.

Activities are the building blocks of a domain. A domain is typically made up of many
activities, connected through the variables they share.

## What an activity defines

* **An equation** — the mathematical relationship, written in TrueMath's
  [writing equations](/authoring/writing-equations).
* **The variables it relates** — and, for each, whether it is **calculable** by this
  activity (that is, whether the activity can solve for it).
* **A title and description** — so the activity's purpose is clear to the people
  reading the domain.

## Solving in more than one direction

An activity is not locked to a single input-output direction. Given a payment, a rate,
and a term, an activity can solve for the loan amount; given a target payment, it can
solve for the affordable purchase price. The author defines which of an activity's
variables are **calculable** — the ones it may solve for; within that set, which
variable is actually computed depends on the values you provide and what you ask to
solve for. This is what lets a domain answer what-if questions naturally rather than
only running a fixed calculation.

## Activity state

An activity can be **active** or **disabled**. Disabled activities remain part of the
domain definition but are not used when running calculations.

<Note>
  You define activities and their equations when [authoring a domain](/authoring/writing-activities).
  To see the activities in a published domain, use
  [`GET /v1/domains/:id/activities`](/api/domains).
</Note>