Prove that the perpendiculars from $D,E,F$ to $BC,CA,AB$ are concurrent
109880
Prove that the perpendiculars from P,Q,R to BC,CA,AB respectively are also concurrent.
53327
$C_2 \times C_2$ and $C_4$
26861
$a+b=c \times d$ and $a\times b = c + d$
118716
If $ABCD$ is a cyclic quadrilateral, then $AC\cdot(AB\cdot BC+CD\cdot DA)=BD\cdot (DA\cdot AB+BC\cdot CD)$
152476
Unique Perpendicular Geodesic
137982
Show that $\textbf{ZF}$ shows $\phi$ is $\Delta_1$
120292
How to formally prove: if $d\mid da+b$, then $d\mid b$?
157660
proof $a$ ,$b$ and $c$ can be sides of a triangle
22856
Ways to go from A to B(proof)
26790
Proving the geometric sum formula by induction
75199
$a \equiv b \pmod n$ and $c\equiv d \pmod n$ implies $ac \equiv bd \pmod n$
31340
Concurrent lines proof for a regular 18-gon
15365
Proving that the triangle inequality holds for a metric on $\mathbb{C}$
124868
Prove that $\frac{bc}{b+c}+\frac{ca}{c+a}+\frac{ab}{a+b}<\frac{a+b+c}{2}$
89493
Prove that circumcentre of $ABH,BCH,CAH$ are the vertices of a triangle congruent to $ABC$
84422
$AB=BA$, $AC=CA$ we need to show $BC=CB$
121880
How to show that $|D_{2n}| = 2n$ via the presentation?