https://tex.z-dn.net/?f=look%5C+at%5C+the%5C+picture%5C%5C%5C%5Ctan%5Calpha%3D%5Cfrac%7BH%7D%7Br%7D%5Cto+H%3Dr%5Ccdot+tan%5Calpha%5C%5C%5C%5Ccot%5Cbeta%3D%5Cfrac%7BH%7D%7Br%7D%5Cto+H%3Dr%5Ccdot+cot%5Cbeta