Understanding Leibniz's Formula for $\pi/4$, geometric proof

Question

I was reading about Leibniz's geometric proof for $\pi/4$ in this wiki-page. I understand this proof almost completely with the exception of one part where it is stated that:

$$dC=\bigtriangleup OPQ=\frac{OR\cdot PQ}{2}=\frac{OR\cdot ds}{2}$$

It wasn't immediately obvious to me why the two triangles I've drawn have the same area?

Edit : I've added and edited a figure which illustrates how I'm looking at the two triangles.

Answer 1

Triangles with same base and same height have equal area.

(proof)

$ds$ is base of the triangle, while $OR$ is altitude from vertex $O$ on extended $PQ$.

Thus $$ \text{Ar.} \triangle= \frac 12 \times \rm{PQ} \times h =\color{blue}{\frac 12 \times ds \times \rm{OR}} $$