Angle pair equationsHSG.CO.C.9Write and solve an equation for complementary, supplementary and vertical angles whose measures are expressions in x.Parallel lines and transversalsHSG.CO.C.9Use corresponding, alternate interior, alternate exterior and same-side interior angles to find missing angles and solve for x.Triangle angle sum and exterior anglesHSG.CO.C.10Use the triangle angle sum (180°) and the exterior angle theorem to find missing angles and solve for x.Triangle midsegment theoremHSG.CO.C.10Use the midsegment theorem: a segment joining the midpoints of two sides is parallel to the third side and half as long.Properties of parallelogramsHSG.CO.C.11Use the facts that opposite sides and opposite angles of a parallelogram are equal, and consecutive angles add to 180°, to solve for x.