English: Straightedge and compass construction of the length ${\displaystyle x={\sqrt {a}}}$, given the length ${\displaystyle a}$ and the unit length. The proof relies on Thales's theorem (blue circle) to obtain a right triangle with hypothenuse length ${\displaystyle a+1}$, and on the Pythagorean theorem to set up equations about its height and side lengths, viz. ${\displaystyle a^{2}+x^{2}={\sqrt {a^{2}+x^{2}}}^{2}}$ (from the left triangle; trivially satisfied), ${\displaystyle 1^{2}+x^{2}={\sqrt {x^{2}+1}}^{2}}$ (from the right triangle; trivially satisfied), and ${\displaystyle {\sqrt {a^{2}+x^{2}}}^{2}+{\sqrt {x^{2}+1}}^{2}=(a+1)^{2}}$ (from the big triangle). Their unique positive solution is ${\displaystyle x={\sqrt {a}}}$.