Rules of Lines and Triangles

angles around a point

∠a +∠b +∠c +∠d = 360°

angles on a straight line / supplementary angles

∠a + ∠b = 180°

vertically opposite angles

∠a = ∠c

complementary angles

∠e + ∠f = 90°

corresponding angles

∠g = ∠h

co-interior angles

∠i + ∠j = 180°

alternate interior angles

∠k = ∠l

acute angles

Less than 90°

obtuse angles

90° < x < 180°

reflex angles

180° < x < 360°

isosceles triangle

scalene triangle

right triangle

equilateral triangle

exterior angles of a triangle

∠a + ∠b = ∠d

angles in a triangle

add up to 180 deg