AP and BQ are altitudes of triangle ABC. Let
<above sym=rarr>OA</above> = p
<above sym=rarr>OB</above> = q
<above sym=rarr>OC</above> = r
1. Find vector expressions for <above sym=rarr>AC</above> and
<above sym=rarr>BC</above> in terms of p, q, and r.
2. Deduce that q x r = p x q = p x r
3. Hence, prove that OC is perpendicular to AB.
Please help me with this vector problem! Thank you!