Angles in a quadrilateral
We can use the knowledge that angles in a triangle add up to 180° to work out the sum of the angles in a quadrilateral. Any quadrilateral can be split into two triangles by joining opposite corners. Each triangle has an angle sum of 180°, and there are two angles, so the total angle sum is 2 x 180° = 360°.
We can use this fact to work out missing angles in a quadrilateral. If we know three angles, we can add these together and subtract from 360° to work out the fourth angle. You will often see "x" or another letter used to represent a missing angle - this is another example of algebra being used to shorten a process. It is easier to write "x" rather than "the missing angle".
A regular quadrilateral (one with all sides the same length and all angles the same size) is called a square, and each angle measures 90°.
A rectangle also has four angles of 90°, but is not regular, because it does not have four sides of equal length.