Geometry & measures · Angles & polygons
Chapter 1 · 4
The idea
Angles in parallel lines
Why corresponding angles are equal, why alternate angles are equal, and why co-interior angles add up to 180° — plus the exact naming examiners accept, and the add-a-parallel-line trick for awkward figures.
A full journey — read it, play with it, work it, then earn real exam marks. Everything stays on the timeline below.
In this lesson — start anywhere
Geometry & measures · Angles & polygons
Angles in parallel lines
Why corresponding angles are equal, why alternate angles are equal, and why co-interior angles add up to 180° — plus the exact naming examiners accept, and the add-a-parallel-line trick for awkward figures.
Why it works
The same slant at both crossings
Parallel lines point in exactly the same direction. So when one straight line (a transversal) crosses two parallel lines, it meets each of them at the same slant. Slide the whole crossing from the first parallel line down to the second — nothing about the angles changes. The angle in the same position at each crossing is the same: corresponding angles are equal.Alternate angles are equal
They are the pair tucked between the parallels on opposite sides of the transversal. You can build this from what you already know: the alternate angle corresponds to one at the other crossing, and that one is vertically opposite the angle you started with. Equal, then equal again — so alternate angles match.Keep reading — free
The rest of the explanation, plus 2 worked examples you step through move by move.
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