Question

Add: 

p (p − q), q (q − r) and r (r − p)

Answer 1

First expression = p (p − q) = p² − pq

Second expression = q (q − r) = q² − qr

Third expression = r (r − p) = r² − pr

Adding the three expressions, we obtain

Therefore, the sum of the given expressions is p² + q² + r² − pq − qr − rp.