Sp=2p(2a+[p−1]d)
Sq=2q(2a+[q−1]d)
Sq=Sp
q(2a+[q−1]d)=p(2a+[p−1]d)
2a(q−p)=d(p2−p−(q2−q))
2a(q−p)−d(p2−q2−(p−q))=0
2a(q−p)−d((p−q)(p+q)−(p−q))=0
2a(q−p)+d((q−p)(p+q)−(q−p))=0
2a(p+q−1)d=0
2(p+q)(2a+(p+q−1)d)=0=Sp+q