Timeline for Proving that a quadrilateral is a parallelogram

9 events

when toggle format	what		by	license	comment
Jun 14, 2022 at 21:21	vote	accept	lorenzo
Dec 16, 2021 at 3:37	comment	added	dxiv		@lorenzo Opposite angles in a parallelogram are equal.
Dec 15, 2021 at 22:57	answer	added	jlammy		timeline score: 0
Dec 15, 2021 at 22:56	comment	added	Intelligenti pauca		Use first criterium given here: mathwarehouse.com/geometry/quadrilaterals/parallelograms/…
Dec 15, 2021 at 22:34	comment	added	lorenzo		@dxiv thank you for your interest in my question; to prove that, it seems to me, I should start by noting the figure in the center is a rectangle and then prove that the two triangles to the sides of the rectangle are congruent: the two triangles have and equal side (to side of the rectangle) and an equal angle (the 90° one) but I can't see a third equal element to prove congruence. Is there another way to show this? Thanks
Dec 15, 2021 at 22:13	comment	added	lorenzo		@user29418 thank you for your interest in my question; unfortunately I cannot use coordinate geometry
Dec 15, 2021 at 21:55	comment	added	user29418		if you can use coordinate geometry, show that the two slopes are equal
Dec 15, 2021 at 21:55	comment	added	dxiv		"remains to prove that PS and RQ are parallel" $\;-\;$ Try to show that $PQ=RS$ instead.
Dec 15, 2021 at 21:45	history	asked	lorenzo	CC BY-SA 4.0