Published by:
CGP EDU Academic Team
Published on: August 13, 2026
Find the co-efficient of x 7 in 
and of x –7 in 
and find the relation between
'a' & 'b' so that these co-efficients are equal. (where a, b ≠ 0).
Text Solution
Verified by ExpertsThe correct answer is:
CHECK THE SOLUTION.
11 C 5 
, 11 C 6 
, a b = 1
Co-efficient of x 7 in 
General Term = 11 C r (ax 2 ) 11–r 
= 11 C r a 11–r b –r x 22–3r
Put 22 – 3r = 7
r = 5 ∴ Co-efficient of x 7 = 11 C 5 a 6 . b –5
Co-efficient of x –7 in 
General Term = 11 C r (ax) 11–r 
= 11 C r a 11–r –r (–1) r x 11–3r
Put 11 – 3r = –7 ⇒ r = 6 ∴ Co-efficient of x –7 = 11 C 6 a 5 b –6
Given that 11 C 5 a 6 b –5 = 11 C 6 a 5 b –6 ⇒ ab = 1
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