Math · 29 · Geometry and Trigonometry

Lines and Angles

Notes

Straight line = $180^\circ$ . Around a point = $360^\circ$ .
Vertical angles (opposite at an X) are equal.
Parallel lines cut by a transversal: only two angle sizes — big and small.
Big = big, small = small, big + small = $180^\circ$ .
Corresponding and alternate interior angles are equal; same-side interior sum to 180°.
Figures are not always to scale — trust the given facts, not the picture.

Supplementary (linear pair)

a+b=180^\circ

Complementary

a+b=90^\circ

Vertical angles

\angle 1=\angle 3,\quad \angle 2=\angle 4

Same-side interior

a+b=180^\circ

Between parallels, same side of transversal

Full circle

360^\circ=2\pi\text{ rad}

On the reference sheet

Assuming lines are parallel because they look parallel — the question must say so.
Setting a big angle equal to a small one instead of adding them to 180°.
Stopping at x when the question asks for the angle's measure.

Label each angle big or small: same size, equal; one of each, they sum to 180°.

10 official SAT questions on Lines and Angles, easiest first.