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CGP EDU Academic Team
Published on: August 14, 2026
If the coefficients of x 3 and x 4 in the expansion of (1 + ax + bx 2 ) (1 – 2x) 18 in powers of x are both zero, then (a, b) is equal to
A
B
C
D
Text Solution
Verified by ExpertsThe correct answer is:
B
(1 + ax + bx 2 )(1 – 2x) 18
coeff of x 3 = 18 C 3 (–2) 3 + a.(–2) 2 . 18 C 2 + b.(–2). 18 C 1 = 0
coeff of x 4 ≡ 18 C 4 (– 2) 4 + a(–2) 3 . 18 C 3 + b(–2) 2 . 18 C 2 = 0
⇒ 51a – 3b = 544 and 32a – 3b = 240
Subtracting we get a = 16
⇒ b = 
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