Difference between revisions of "2018 AMC 10B Problems/Problem 11"
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==Solution 3== | ==Solution 3== | ||
− | From Fermat's Little Theorom, <math>p^2 \equiv 1\pmod 3</math> if <math>p</math> is | + | From Fermat's Little Theorom, <math>p^2 \equiv 1\pmod 3</math> if <math>p</math> is coprime with <math>3</math>. |
So for any <math>n\equiv2\pmod3</math>, <math>p^2+n \equiv 0\pmod 3</math> - divisible by 3, so not a prime. | So for any <math>n\equiv2\pmod3</math>, <math>p^2+n \equiv 0\pmod 3</math> - divisible by 3, so not a prime. | ||
The only choice <math>\equiv2\pmod3</math> is <math>\boxed{(\textbf{C})}</math> | The only choice <math>\equiv2\pmod3</math> is <math>\boxed{(\textbf{C})}</math> |
Revision as of 21:04, 22 February 2018
Which of the following expressions is never a prime number when is a prime number?
Solution 1
Because squares of a non-multiple of 3 is always , the only expression is always a multiple of is . This is excluding when , which only occurs when , then which is still composite.
Solution 2 (Bad Solution)
We proceed with guess and check: . Clearly only is our only option left. (franchester)
Solution 3
From Fermat's Little Theorom, if is coprime with . So for any , - divisible by 3, so not a prime. The only choice is
Solution 4
Primes can only be or . Therefore, the square of a prime can only be . then must be , so it is always divisible by . Therefore, the answer is .
See Also
2018 AMC 10B (Problems • Answer Key • Resources) | ||
Preceded by Problem 10 |
Followed by Problem 12 | |
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 |
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