Isosceles triangles and bisectors
Base angles equal; perpendicular bisector = points equidistant from the ends.
Watch out: halve 180° − apex for each base angle.
Revision sheet
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Parallelograms
Opposite sides and angles equal; consecutive angles 180°; diagonals bisect each other.
Watch out: opposite (equal) vs consecutive (supplementary).
Polygon angles
Interior sum (n − 2) · 180°; exterior sum 360°.
Watch out: n − 2 triangles, not n.
Congruence proofs
Facts with reasons → criterion → conclusion → corresponding parts.
Watch out: never use what you are proving as a reason.
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