Angles

Angle problems reward students who know a handful of facts cold and can say which fact they are using. This topic collects the ones that come up most. It begins with the angles inside a triangle adding to 180 degrees, and the common slip of forgetting that an isosceles triangle hands you a second equal angle for free. Next are angles formed where a line crosses two parallel lines: corresponding, alternate and co-interior angles. Here the mistake is usually calling two angles equal when they actually add to 180. The last page covers the exterior angle of a triangle, which equals the sum of the two opposite interior angles. That shortcut saves a step, but many students subtract instead of add. Draw a sketch for every question, label what you know, and write the reason next to each angle you find. Showing the reason is often worth marks on its own, and it makes a wrong step easy to spot when you check your work.