Angle-angle (AA) similarity
SAT glossary · Course Foundations
Definition
Two triangles with two pairs of equal angles are similar, since the third pair must then match by the 180^circ sum. It is almost the only way the test establishes similarity — look for shared angles and for parallel lines producing equal angles.
Angle-angle (AA) similarity in plain English
Similar triangles are triangles that have the exact same shape but might be different sizes — like a photo and a zoomed-in copy of it. One reliable way to prove two triangles are similar is to show that two of their angles match up exactly. Why does that work? Because every triangle's three angles always add up to 180°, so if two angles already match between two triangles, the third angle HAS to match too (it's forced by subtraction) — meaning all three angles line up and the shapes are guaranteed similar. On the test, you'll often spot two matching angles because the triangles share an angle, or because two parallel lines create equal angles when a third line crosses them.
Also called: AA
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