Difference between revisions of "Mock AIME 1 2007-2008 Problems/Problem 12"
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== Solution == | == Solution == | ||
<cmath>\begin{align*}[(a^2 + 2^a) + a \cdot 2^{(a+1)/2}][(a^2 + 2^a) - a \cdot 2^{(a+1)/2}] &= (a^2 + 2^a)^2 - a^2 \cdot 2^{a+1}\\ | <cmath>\begin{align*}[(a^2 + 2^a) + a \cdot 2^{(a+1)/2}][(a^2 + 2^a) - a \cdot 2^{(a+1)/2}] &= (a^2 + 2^a)^2 - a^2 \cdot 2^{a+1}\\ | ||
− | &= a^4 + 2 \cdot a^ | + | &= a^4 + 2 \cdot a^22^{a} + 2^{2a} - a^2 \cdot 2^{a+1}\\ |
&= a^4 + 2^{2a}\end{align*}</cmath> | &= a^4 + 2^{2a}\end{align*}</cmath> | ||
− | (If you recall the reverse of [[Sophie Germain Identity]] with <math>a=a,\, b = 2^{a/2}</math>, then you could have directly found the answer). | + | (If you recall the reverse of [[Sophie Germain Identity]] with <math>a=a,\, b = 2^{(a-1)/2}</math>, then you could have directly found the answer). |
By [[Fermat's Little Theorem]], we have that <math>a^{4} \equiv 1 \pmod{5}</math> if <math>a \nmid 5</math> and <math>a^{4} \equiv 0 \pmod{5}</math> if <math>a | 5</math>. Also, we note that by examining a couple of terms, <math>2^{2a} \equiv 4 \pmod{5}</math> if <math>a \nmid 2</math> and <math>2^{2a} \equiv 1 \pmod{5}</math> if <math>a|2</math>. Therefore, | By [[Fermat's Little Theorem]], we have that <math>a^{4} \equiv 1 \pmod{5}</math> if <math>a \nmid 5</math> and <math>a^{4} \equiv 0 \pmod{5}</math> if <math>a | 5</math>. Also, we note that by examining a couple of terms, <math>2^{2a} \equiv 4 \pmod{5}</math> if <math>a \nmid 2</math> and <math>2^{2a} \equiv 1 \pmod{5}</math> if <math>a|2</math>. Therefore, |
Latest revision as of 12:08, 10 March 2012
Problem 12
Let and
. If
, how many integral values of
are there such that
is a multiple of
?
Solution
(If you recall the reverse of Sophie Germain Identity with , then you could have directly found the answer).
By Fermat's Little Theorem, we have that if
and
if
. Also, we note that by examining a couple of terms,
if
and
if
. Therefore,
With divisibility by
achievable only if
. There are
odd numbers in the range given, and
of those are divisible by
, so the answer is
.
See also
Mock AIME 1 2007-2008 (Problems, Source) | ||
Preceded by Problem 11 |
Followed by Problem 13 | |
1 • 2 • 3 • 4 • 5 • 6 • 7 • 8 • 9 • 10 • 11 • 12 • 13 • 14 • 15 |