Finding Coefficients

Find the coefficient of x x in the expansion of ( x + 2 x ) 9 \large \left(x+\frac{2}{x}\right)^{9}

2017 2015 2016 2013

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Tapas Mazumdar
Mar 4, 2017

The ( r + 1 ) th (r+1) \text{th} term of the expansion can be given by

T r + 1 = ( 9 r ) x 9 r { 2 x } r T_{r+1} = \dbinom 9r x^{9-r} {\left\{ \dfrac 2x \right\}}^r

Here, we see that the exponent of x x is 9 2 r 9-2r .

Thus, 9 2 r = 1 r = 4 9-2r = 1 \implies r = 4 .

\therefore Coefficient of x x is ( 9 4 ) 2 4 = 2016 \dbinom 94 \cdot 2^4 = \boxed{2016} .

Nice thought.

Hana Wehbi - 4 years, 3 months ago
Hana Wehbi
Jun 9, 2016

The expansion of x \large x in ( x + 2 x ) 9 = x 9 + 512 x 9 + 18 x 7 + 2304 x 7 + 144 x 5 + 4608 x 5 + 672 x 3 + 5376 x 3 + 2016 x + 4032 x (\Large x+\frac{2}{x})^{9} = x^{9}+ \frac{512}{x^{9}} + 18x^{7}+\frac{2304}{x^{7}}+144x^{5}+\frac{4608}{x^{5}}+672x^{3}+\frac{5376}{x^{3}} + \color{#3D99F6}{2016}x+ \frac{4032}{x}

Change the options... easily you can deduce that the coefficient of x x must be an even number, the only such option is 2016.

Mateo Matijasevick - 5 years ago

Log in to reply

Exactly, that's exactly how I figured it out :p .Juz coz i'm lazy

Mehul Arora - 5 years ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...