The most common shape that you work with in
geometry is the triangle. A triangle has three corner points and three
straight sides, and must have an area. 3 points in a straight line *do not*
form a triangle:

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A triangle has to have some area – the line just
shown has no area. If you make sure that the points *do not* form a
straight line, you will get a shape with area – a triangle:

### Types of triangles

Like angles, there are different *types* of
triangles. You can describe types of triangles in two different ways. The
first way is to look at the size of the *interior* *angles* in the
triangle and describe it using them. The other way of describing triangles is
to talk about them by looking at the *sides* of the triangle. Here I’ll go
through all the different types of triangles, either described using angles or
using sides.

Handy Hint #1 - Angles within a triangle rule

If you add up the three interior
angles inside **any** triangle, they **always** add up to 180 degrees:

This means that for **any**
triangle, such as the one shown above, the following rule is true:

_{}

Say you were given a triangle like the following one and asked to find what ‘a’ was:

All you have to remember is that the sum of all the interior angles of a triangle is 180°. So you could write down a simple mathematical equation:

_{}

To work out what ‘a’ is you just need
to simplify the equation and then work out what value of ‘a’ makes it *true*.

_{}

Subtract 104° from both sides:

_{}

And there you have your answer – a is 76°.