Apply the Triangle Inequality Theorem and the side-angle ordering relationship, apply the Midsegment Theorem, and review the semester's major proof tools.
The Triangle Inequality Theorem states that the sum of the lengths of any two sides of a triangle must be greater than the length of the third side, true for all three pairs at once -- exactly the condition deciding whether three given lengths can form a triangle at all. A related fact links sides and angles directly: in any triangle, the longest side is always opposite the largest angle, and the shortest side is always opposite the smallest angle.
The Midsegment Theorem describes a segment connecting the midpoints of two sides of a triangle: that midsegment is always parallel to the third side, and always exactly HALF its length. As the semester closes, the real skill worth reviewing is not any one formula but choosing the RIGHT tool for a triangle problem -- Triangle Sum, exterior angles, a congruence shortcut, the Base Angles Theorem, a point of concurrency, or this week's ideas -- since most real problems do not announce which chapter they came from.
Mastery looks like: They correctly apply the Triangle Inequality Theorem to determine whether three lengths form a triangle, order a triangle's sides and angles consistently, and apply the Midsegment Theorem.
Common stumbles: Checking only one pair of sides instead of the pair involving the longest side; forgetting the midsegment is HALF the third side, not equal to it.