Let a1, a2, a3, ... be in AP and ap, aq , ar be in GP. Then aq : ap is equal to
Let
ap = a1 + (p - 1)d, aq = a1 + (q - 1)d, ar = a1 + (r - 1)d
as ap, aq, ar are in G.P.
aqap = araq = aq−arap−aq (by law of proportions)
or aqap = araq = a1+(q−1)d−a1−(r−1)da1+(p−1)d−a1−(q−1)d = q−rp−q or aqap = q−rp−q = r−qq−p