Coefficient

Algebra Level 3

Find the absolute value of the coefficient of 1 x \dfrac{1}{x} in th expression of: ( 2 x 2 1 x ) 10 \large\left(2x^2-\dfrac{1}{x}\right)^{10}


This is a part of the Set .


The answer is 960.

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1 solution

Answer is 960 960 .

We have: ( 2 x 2 1 x ) 10 = k = 0 10 ( 10 k ) ( 2 x 2 ) 10 k . ( 1 x ) k \displaystyle\left(2x^2-\dfrac{1}{x}\right)^{10}=\sum_{k=0}^{10}\binom{10}{k}\left(2x^2\right)^{10-k}.\left(-\dfrac{1}{x}\right)^k

= k = 0 10 ( 10 k ) 2 10 k ( 1 ) k . x 20 3 k \qquad\qquad\qquad\qquad\qquad\displaystyle=\sum_{k=0}^{10}\binom{10}{k}2^{10-k}(-1)^k.x^{20-3k}

If 20 3 k = 1 20-3k=-1 , then k = 7 k=7 .

So, the cofficient of 1 x \dfrac{1}{x} is ( 10 7 ) 2 10 7 ( 1 ) 7 = 960 \displaystyle\binom{10}{7}2^{10-7}(-1)^7=-960 .

And the absolute value of it is 960 \boxed{960} .

Coefficient of 1/x is (10C3)2^3

Sid Rana - 4 years, 1 month ago

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Coefficient of 1/x is (10C8)2^3

Sid Rana - 4 years, 1 month ago

Coefficient of 1/x is (10C3)2^3 is wrong

Sid Rana - 4 years, 1 month ago

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