Two angles are supplementary. If one is 75°, what is the other?

Supplementary angles sum to 180°. So the other angle = 180° − 75° = 105°. Angle relationships to know: (1) Complementary — sum to 90° (think 'right angle'); (2) Supplementary — sum to 180° (think 'straight angle'); (3) Vertical (vertically opposite) — formed by intersecting lines; vertical angles are equal; (4) Adjacent — share a vertex and side, don't overlap; (5) Linear pair — adjacent angles forming a straight line; sum to 180° (always supplementary). Parallel lines cut by a transversal: (1) Corresponding angles — equal; (2) Alternate interior angles — equal; (3) Alternate exterior angles — equal; (4) Co-interior (same-side interior) angles — supplementary. Angle measurement: degrees (most common; circle = 360°), radians (advanced; 2π radians = 360°). Acute < 90°; right = 90°; obtuse 90°-180°; straight = 180°; reflex 180°-360°.