Proof that a circle can be circumscribed around a quadrilateral
158727
Leg bisectors in a trapezoid and angle equality
101493
Prove that one diagonal of a quadrilateral bisects the other
63571
The Mississipi counting problem, this time in circle
13136
Element chasing proof that $(A\setminus C)\setminus(B\setminus C) = (A\setminus B)\setminus C$
111032
Proof of Sets involving the Cartesian Product
76490
Proving that $\cos(\arcsin(x))=\sqrt{1-x^2}$
62755
The simplest way of proving that $|\mathcal{P}(\mathbb{N})| = |\mathbb{R}| = c$
72707
Property of Medians and Cirumcircle
31318
Cardinality of cartesian square
44980
Prove that two paths on opposing corners of the unit square must cross.
149543
Prove that an annulus is not simply connected?
148233
Proof that every circle has the ratio of $\pi$
134652
Prove that the perpendicular bisectors of the sides of a triangle meet at a point
41907
Triangulation of a simple polygon (elementary proof?)
147861
Packing squares into a circle
142142
Show that a convex polygon is contained within the largest circle determined by three consecutive vertices
69631
Prove that: $ \cot7\frac12 ^\circ = \sqrt2 + \sqrt3 + \sqrt4 + \sqrt6$