Difference between revisions of "2016 AMC 10B Problems/Problem 25"
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<math>\textbf{(A)}\ 32\qquad\textbf{(B)}\ 36\qquad\textbf{(C)}\ 45\qquad\textbf{(D)}\ 46\qquad\textbf{(E)}\ \text{infinitely many}</math> | <math>\textbf{(A)}\ 32\qquad\textbf{(B)}\ 36\qquad\textbf{(C)}\ 45\qquad\textbf{(D)}\ 46\qquad\textbf{(E)}\ \text{infinitely many}</math> | ||
+ | |||
+ | ==Solution== | ||
+ | Since <math>x = \lfloor x \rfloor + \{ x \}</math>, we have | ||
+ | |||
+ | <cmath>f(x) = \sum_{k=2}^{10} (\lfloor k \lfloor x \rfloor + k \{ x \} \rfloor - k \lfloor x \rfloor)</cmath> | ||
+ | |||
+ | The function can then be simplified into | ||
+ | |||
+ | <cmath>f(x) = \sum_{k=2}^{10} ( k \lfloor x \rfloor + \lfloor k \{ x \} \rfloor - k \lfloor x \rfloor)</cmath> | ||
+ | |||
+ | which becomes | ||
+ | |||
+ | <cmath>f(x) = \sum_{k=2}^{10} \lfloor k \{ x \} \rfloor</cmath> | ||
+ | |||
+ | We can see that for each value of k, <math>\lfloor k \{ x \} \rfloor</math> can equal integers from 0 to k-1. | ||
+ | |||
+ | Clearly, the value of <math>\lfloor k \{ x \} \rfloor</math> changes only when x is equal to any of the fractions <math>\frac{1}{k}, \frac{2}{k} \dots \frac{k-1}{k}</math>. | ||
+ | |||
+ | So we want to count how many distinct fractions have the form <math>\frac{m}{n}</math> where <math>n \le 10</math>. We can find this easily by computing | ||
+ | <cmath>\sum_{k=2}^{10} \phi(k)</cmath> | ||
+ | where <math>\phi(k)</math> is the Euler Totient Function. Basically <math>\phi(k)</math> counts the number of fractions with <math>k</math> as its denominator (after simplification). This comes out to be <math>31</math>. | ||
+ | |||
+ | Because the value of <math>f(x)</math> is at least 0 and can increase 31 times, there are a total of 32 different possible values of <math>f(x)</math>. | ||
==See Also== | ==See Also== | ||
{{AMC10 box|year=2016|ab=B|num-b=24|after=Last Problem}} | {{AMC10 box|year=2016|ab=B|num-b=24|after=Last Problem}} | ||
{{MAA Notice}} | {{MAA Notice}} |
Revision as of 20:45, 26 September 2016
Problem
Let , where
denotes the greatest integer less than or equal to
. How many distinct values does
assume for
?
Solution
Since , we have
The function can then be simplified into
which becomes
We can see that for each value of k, can equal integers from 0 to k-1.
Clearly, the value of changes only when x is equal to any of the fractions
.
So we want to count how many distinct fractions have the form where
. We can find this easily by computing
where
is the Euler Totient Function. Basically
counts the number of fractions with
as its denominator (after simplification). This comes out to be
.
Because the value of is at least 0 and can increase 31 times, there are a total of 32 different possible values of
.
See Also
2016 AMC 10B (Problems • Answer Key • Resources) | ||
Preceded by Problem 24 |
Followed by Last Problem | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 • 16 • 17 • 18 • 19 • 20 • 21 • 22 • 23 • 24 • 25 | ||
All AMC 10 Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.