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!