If the constant term in the binomial expansion of x - k x 2 10 is 405 , then k equals :

Question

If the constant term in the binomial expansion of x - k x 2 10 is 405 , then k equals :, 9, 1, 3, 2

MARKS App · Accepted Answer

T r + 1 = C r 10 - k x 2 r x 10 - r = C r · 10 - k r · x 5 - 5 r 2 for constant term ⇒ 5 - 5 r 2 = 0 ⇒ r = 2 ⇒ T 3 = C 2 10 · k 2 = 405 ⇒ 10 ( 9 ) 2 k 2 = 405 ⇒ k 2 = 9 ⇒ k = 3

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