Geometric proof · Grade 11
In Grade 11 you learn: geometric proofs.
A geometric proof shows that something is always true, step by step, giving a reason for every line. The reasons are the angle facts you know: angles on a line, vertically opposite angles, parallel lines, triangles, isosceles triangles and circle theorems.
Key facts
- Every statement needs a reason, written in words (for example “alternate angles are equal”).
- Angles on a straight line add to 180°; round a point to 360°; vertically opposite angles are equal.
- Parallel lines: alternate and corresponding angles are equal; co-interior angles add to 180°.
- Triangle angles add to 180°; base angles of an isosceles triangle are equal; the exterior angle equals the two opposite interior angles.
- A single counter-example is enough to show a claim is false.