The following triangle ABC, has a right-angle at B. The line BX is perpendicular to AC.
angle ABX is α and the angle at XBC is β. Because XBC is a right-angled
triangle by construction, then angle C is 90°−angle XBC (β),
so angle C is α. By a similar argument, angle A is β, and these are
marked on the diagram above. The length of BX is h.