Angles on a Straight Line

Angles on a Straight Line

Angles that lie on a straight line always add up to 180°. This is the key fact used in these questions.

1) Identify the known angles

Look at the diagram and note which angles are given. If one angle is missing, call it something like (x).

2) Write an equation

Since angles on a straight line sum to 180°, add the known angles and set the total equal to 180°.

• If two angles are on the line: (x + a = 180)
• If more than two angles are shown: add them all and use the same total.

3) Solve for the unknown

Rearrange the equation to find the missing angle. If the answer includes numbers or expressions, simplify fully.

4) Check your result

Add your answer back to the other angle(s). The total should be 180°. If it is not, recheck the arithmetic.

Helpful reminder

Make sure you are only using angles that are directly on the same straight line. Angles around a point or inside a shape use different rules.

Example method

If one angle is (125°), the missing angle is: [ 180° - 125° = 55° ] So the answer is (55°).