What does ∆ mean in this case?

[Herricane]

1. Problem

Use any method you wish to verify the following identities:
a. (A ∆ B) U C = (A U C) ∆ (B \ C)


2. Relevant equations
None


3. The attempt at a solution

I don't understand what this symbol means ∆. It is definitely not the rate of change.

[micromass]

Usually, it denotes the following:

A\Delta B= (A\setminus B)\cup (B\setminus A)

[Herricane]

Alright thanks.