# Basic geometry terms for OCR GCSE Maths 1. Angles at a point
2. Angles at a point on a straight line
3. Vertically opposite angles
4. Alternate angles
5. Corresponding angles

The sum of the angles at a point is 360º. This property can be used to find the value of missing angles. The sum of the angles at a point on a straight line is 180º. This property can be used to find the value of missing angles. Vertically opposite angles are angles which are opposite each other at a point. These angles are equal. Given two parallel lines, the marked angles on the line joining the two parallel lines are equal. These angles are called alternate angles or Z angles. The angles between two parallel lines and a third intersecting lines, as shown on the diagram, are equal. These angles are called corresponding angles or F angles. # 1

State all the pairs of vertically opposite angles on the given diagram.

The pairs of vertically opposite angles are a and d, b and e, and c and f. # 2

Calculate the value of y.

The 98º angle and y are corresponding angles, therefore they are equal and y = 98º. # 3

Calculate the value of angle a.

a = 360 − (15 + 95 + 10 + 15) = 225º. # 4

Find the value of b, given that d = 78º and f = 83º.

Using the sum of angles on a straight line, e = 180 − (78 + 83) = 19º.
Angle b is the vertically opposite angle to e, therefore b = e = 19º. # 5

Use alternate angles to find the value of θ.

The angle next to the 112º angle is 180 − 112 = 68º.
This is an alternate angle to θ, therefore the two are equal and a = 68º. End of page