The angles in every triangle, what they make it, and the pairs that let you find the ones not marked.
The answer key prints on a separate page.
The angles of a triangle sum to 180°, and that one fact does most of the work on this paper: it turns two known angles into the third and tells you what a triangle is before any side is measured. The reliable error across the whole unit is trusting the picture — an angle that looks like a right angle is not one until the arithmetic says so, and a figure drawn a little off is exactly where that assumption gets caught.
Angle pairs and parallel-line angles extend the same reasoning to the angles a figure does not label. Complementary and supplementary pairs, vertical angles, and the equal-and-supplementary angles a transversal cuts across parallel lines are all ways of finding a missing measure from a known one. A child who can classify a triangle but stalls on the parallel-line section has the angle sum and not yet the idea that angles come in fixed relationships.
