Let the two quantities be a and b. Then a,A1,A2,b are in AP. ∴A1−a=b−A2 ⇒A1+A2=a+b…(i)
Again a,G1,G2,b are in GP. ∴aG1=G2b ⇒G1G2=ab…(ii)
Also, a,H1,H2,b are in HP. ∴H11−a1=b1−H21 ⇒H21+H21=b1+a1 ⇒H1H2H1+H2=aba+b ⇒H1H2H1+H2=G1G2A1+A2 [using Eqs. (i) and (ii)] ⇒H1H2G1G2=H1+H2A1+A2