Angles inside a shape are called interior angles. Interior angles in a triangle add up to 180°. Interior angles in a quadrilateral add up to 360°.

The three angles in any triangle add up to 180°.

You can use this information to find a missing angle.

A scalene triangle has three different angles. To find a missing angle you need to know the other two angles.

\[x^\circ = 180^\circ - (70^\circ + 50^\circ )\]

\[= 180^\circ - 120^\circ = 60^\circ\]

An isosceles triangle is one with two sides equal in length and two equal angles.

You only need to know one angle in an isosceles triangle to work out the other two.

- Question
In the diagram below the triangle is isosceles. What is the value of \(a^\circ\)?

\[a^\circ = 180^\circ - (70^\circ + 70^\circ )\]

\[= 180^\circ - 140^\circ = 40^\circ\]

- Question
What is the size of angle \({p}\)?

This is an isosceles triangle, so both the bottom angles are \({p}\).

The angles in a triangle add up to \({180}^\circ\), so:

\[p + p + 40 = 180\]

\[2p + 40 = 180\]

\[2p = 140\]

\[p = 70\]

So the missing angles are both \(70^\circ\).

An equilateral triangle is one in which all three angles are equal.

The angles add up to 180°, so each angle is 60°.

You don't need to be told any angles in an equilateral to find a missing angle. The angles are always 60°.