If S1, S2, S3 be respectively the sums of n, 2n, 3n terms of a G.P., then prove that S12 + S22 = S1(S2 + S3)
Question May be wrong.
Sum of GP for n terms S1 =
= …(1)
Sum of GP for 2n terms S2 =
= …(2)
Sum of GP for 3n terms S3 = …(3)
=
Let
∴ 1, 2 and 3 becomes
S1 = K(rn - 1)
S2 = K(r2n - 1)
S3 = K(r3n - 1)
∴ S12 + S22 = k2(rn - 1)2 + k2(r2n - 1)2
= k2(r2n + 1 - 2.rn + r4n + 1 - 2.r2n)
= k2(r4n - r2n - 2rn + 2)
L.H.S = k2(r4n - r2n - 2rn + 2)
S1(S2 + S3) = K(rn - 1)[ (K(r2n - 1) + K(r3n - 1))]
= K2(rn - 1)[r2n + r3n - 2]
= k2(r4n - r2n - 2rn + 2)
Hence, Proved.