NCERT Solutions
Class 11 Maths
Sequences and Series

Ex.9.3 Q.25
If a, b, c and d are in G.P. show that: (a2 + b2 + c2) (b2 + c2 + d2) = (ab + bc – cd)2
Given, a, b, c, d is in G.P. Therefore,
bc = ad ........................ (1)
b2 = ac ........................ (2)
c2 = bd ....................... (3)
It has to be proved that,
(a2 + b2 + c2) (b2 + c2 + d2) = (ab +bc– cd)2
R.H.S. = (ab + bc + cd)2
= (ab + ad + cd)2
[Using equation (1)]
= [ab + d (a + c)]2
= a2b2 + 2abd (a + c) + d2(a + c)2
= a2b2 +2a2bd + 2acbd + d2 (a2 + 2ac + c2)
= a2b2 + 2a2c2+ 2b2c2+ d2a2 + 2d2b2 + d2c2
[Using equation (1) and (2)]
= a2b2 + a2c2 + a2c2 + b2c2 + b2c2 + d2a2 + d2b2 + d2b2 + d2c2
= a2b2 + a2c2 + a2d2 + b2 × b2 + b2c2 + b2d2 + c2b2 + c2 × c2 + c2d2
[Using (2) and (3) and rearranging terms]
= a2 (b2 + c2 + d2) + b2(b2 + c2 + d2) + c2(b2 + c2 + d2)
= (a2 + b2 + c2) (b2 + c2 + d2) = LHS
So, LHS = RHS
Hence, (a2 + b2 + c2) (b2 + c2 + d2) = (ab + bc – cd)2