"Quadrature of the Parabola". Archimedes. pp. 336–345.{{cite web}}: CS1 maint: url-status (link) ↑ The green triangle has half of the width of blue triangle by construction. The statement about the height follows from the geometric properties of a parabola, and is easy to prove using modern analytic geometry. ↑ Strictly speaking, Archimedes evaluates the partial sums of this series, and uses the Archimedean property to argue that the partial sums become arbitrarily close to 4/3.