I tried doing a search for this one here, but I didn't seem to come up with much...

**1. The problem statement, all variables and given/known data**

Prove that the three altitudes of a triangle meet at one point.

**2. Relevant equations**

Well, I am not really sure, but I know that Ceva's theorem might help me prove this, but I want to prove it vectorally, so I don't know if it will be much help at all.

**3. The attempt at a solution**

My instructor gave a hint for this proof by saying, show each of the triangles cevians are perpandicular to its opposite side... I can see pictorally this is ok, but where to start...???

Thanks