Equal Parts of a Corner

Equal angles carry the same mark: one small arc each, or two arcs each for a second equal pair. When two equal parts share a letter, the letter appears twice in the equation, and x + x is 2x.

Here a right angle is cut into three parts: two equal parts of x° and one of 20°. The parts fill the corner, so $x+x+20 = 90$. Collect the x terms to get $2x+20 = 90$, then $2x = 70$ and $x = 35$.

The arcs are why both parts may carry the same letter. Without them, two unknown parts would need two different letters, and one equation could not fix both.