Difference between revisions of "Mock AIME 1 2007-2008 Problems/Problem 6"
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== Solution == | == Solution == | ||
Note that the value in the <math>r</math>th row and the <math>c</math>th column is given by <math>\left(\frac{1}{(2p)^r}\right)\left(\frac{1}{p^c}\right)</math>. We wish to evaluate the summation over all <math>r,c</math>, and so the summation will be, using the formula for an infinite [[geometric series]]: | Note that the value in the <math>r</math>th row and the <math>c</math>th column is given by <math>\left(\frac{1}{(2p)^r}\right)\left(\frac{1}{p^c}\right)</math>. We wish to evaluate the summation over all <math>r,c</math>, and so the summation will be, using the formula for an infinite [[geometric series]]: | ||
− | < | + | <cmath>\begin{align*}\sum_{r=1}^{\infty}\sum_{c=1}^{\infty} \left(\frac{1}{(2p)^r}\right)\left(\frac{1}{p^c}\right) &= \left(\sum_{r=1}^{\infty} \frac{1}{(2p)^r}\right)\left(\sum_{c=1}^{\infty} \frac{1}{p^c}\right)\\ |
&= \left(\frac{1}{1-\frac{1}{2p}}\right)\left(\frac{1}{1-\frac{1}{p}}\right)\\ | &= \left(\frac{1}{1-\frac{1}{2p}}\right)\left(\frac{1}{1-\frac{1}{p}}\right)\\ | ||
− | &= \frac{2p^2}{(2p-1)(p-1)}\end{align*}</ | + | &= \frac{2p^2}{(2p-1)(p-1)}\end{align*}</cmath> |
Taking the denominator with <math>p=2008</math> (indeed, the answer is independent of the value of <math>p</math>), we have <math>m+n \equiv 2008^2 + (2008-1)(2\cdot 2008 - 1) \equiv (-1)(-1) \equiv 1 \pmod{2008}</math> (or consider [[FOIL]]ing). The answer is <math>\boxed{001}</math>. | Taking the denominator with <math>p=2008</math> (indeed, the answer is independent of the value of <math>p</math>), we have <math>m+n \equiv 2008^2 + (2008-1)(2\cdot 2008 - 1) \equiv (-1)(-1) \equiv 1 \pmod{2008}</math> (or consider [[FOIL]]ing). The answer is <math>\boxed{001}</math>. | ||
Latest revision as of 17:35, 6 May 2024
what the sigma
Solution
Note that the value in the th row and the
th column is given by
. We wish to evaluate the summation over all
, and so the summation will be, using the formula for an infinite geometric series:
Taking the denominator with
(indeed, the answer is independent of the value of
), we have
(or consider FOILing). The answer is
.
With less notation, the above solution is equivalent to considering the product of the geometric series . Note that when we expand this product, the terms cover all of the elements of the array.
By the geometric series formula, the first series evaluates to be . The second series evaluates to be
. Their product is
, from which we find that
leaves a residue of
upon division by
.
See also
Mock AIME 1 2007-2008 (Problems, Source) | ||
Preceded by Problem 5 |
Followed by Problem 7 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 |