Coefficients Party, Part 5

Algebra Level 4

How many terms in the expansion of ( 2 x + 3 y ) 2014 (2x+3y)^{2014} are divisible by 12 ? 12?

2011 2014 2013 2012

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3 solutions

Trevor B.
May 8, 2014

Any number divisible by 12 12 is divisible by 2 2 × 3. 2^2\times3. There are three places where this isn't achieved by direct application of the powers of the coefficients in the expression: the x 2014 x^{2014} term, the y 2014 y^{2014} term, and the x y 2013 xy^{2013} term. You might think the answer is 12 , 12, but remember that because of the Binomial Theorem, the coefficient of the x y 2013 xy^{2013} term is ( 2014 2013 ) × 2 × 3 2013 = 4028 × 3 2013 . \binom{2014}{2013}\times2\times3^{2013}=4028\times3^{2013}. This is divisible by 12 , 12, so the only two terms that aren't divisible by 12 12 are the x 2014 x^{2014} term and the y 2014 y^{2014} term. Subtracting this from the 2015 2015 terms in the expression, there are 2013 \boxed{2013} terms with coefficients divisible by 12. 12.

I didnt consider the (x)(y)^2013 term. at all. Got a little lucky then.. :P

Mudit Jha - 6 years, 10 months ago

no of term will be n+1 ..... and first and last term is not divided by 12 so remaining term are 2013.

Arjun Chotaliya
May 11, 2014

its so simple its made easy

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