Newton's Nightmares

Algebra Level 5

Determine for what value of x x is the difference between 9 9 times the third term of binomial expansion of ( 2 x 1 2 3 + 4 3 2 x 2 ) m \left( \frac{\sqrt{2^{x-1}}}{\sqrt[3]{2}}+\sqrt[3]{4}\cdot 2^{\frac{x}{2}}\right) ^m and the fifth term of the same expansion equal to 240 240 , if it is known, that the difference between the log of the tripled binomial coefficient of the fourth term of expansion and the log of the binomial coefficient of the second term is equal to 1 1 .


The answer is 2.

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2 solutions

Mayank Singh
Mar 12, 2015

The question isn't tricky

All that it requires is a determination

John M.
Oct 22, 2014

By the condition,

log ( 3 C m 3 ) log C m 1 = 1 , \log{(3\cdot C_{m}^3)}-\log{C_{m}^1}=1,

or

log 3 C m 3 C m 1 = log 10 ; \log{\frac{3C_{m}^3}{C_{m}^1}}=\log{10};

from here,

3 C m 3 C m 1 = 10. \frac{3C_{m}^3}{C_{m}^1}=10.

Simplifying, we obtain the expression m 2 3 m 18 = 0 m^2-3m-18=0 , whose roots we get to be m 1 = 6 m_1=6 and m 2 = 3 m_2=-3 . The root appropriate to the binomial is m = 6 m=6 .

From the condition that 9 T 3 T 5 = 240 9T_3-T_5=240 we obtain the equation

9 C 6 2 2 2 ( 2 3 + x 2 ) 2 4 ( x 1 2 1 3 ) C 6 4 2 4 ( 2 3 + x 2 ) 2 2 ( x 1 2 1 3 ) = 240 , 9C_{6}^22^{2(\frac{2}{3}+\frac{x}{2})}2^{4(\frac{x-1}{2}-\frac{1}{3})}-C_{6}^42^{4(\frac{2}{3}+\frac{x}{2})}2^{2(\frac{x-1}{2}-\frac{1}{3})}=240,

from which we find

9 2 3 x 2 2 3 x + 1 = 16 , 9\cdot 2^{3x-2}-2^{3x+1}=16,

or

9 2 3 x 2 2 2 3 x 2 = 16. \frac{9\cdot 2^{3x}}{2^2}-2^{3x}\cdot2=16.

And so,

2 3 x = 2 6 2^{3x}=2^6

thus,

x = 2 \boxed{x=2}

You're no fun.

Jake Lai - 6 years, 1 month ago

ur first +1 !!

A Former Brilliant Member - 6 years, 2 months ago

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