In $\triangle ABC$ with $\angle C$ acute, $BC = a$, $AC = b$, $CD$ is the orthogonal projection of $AC$ on $BC$, and $CE$ is the orthogonal projection of $BC$ on $AC$. Then —

• a$a \cdot CD = b \cdot CE$
• b$a \cdot CE = b \cdot CD$
• c$a \cdot b = CD \cdot CE$
• d$a + b = CD + CE$