1. gradient of PQ = (ap^2 - aq^2) / (2ap - 2aq)
= [ a (p-q)(p+q) ] / [ 2a (p-q) ]
= p+q / 2
= m
that is, (p+q)/2 = m
now the midpoint of PQ,
x = (2aq + 2aq) / 2
= a (p+q)
y = (ap^2 + aq^2) / 2
= a (p^2+q^2) / 2
Now, x = a (p+q), but p+q = 2m
therefore, x = 2am, which is a constant
and...