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Difference between revisions of "2011 AMC 10B Problems/Problem 23"
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== Solution 3 ==
 
== Solution 3 ==
  
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Notice that the hundreds digit of <math>2011^{2011}</math> won't be affected by <math>2000</math>. Essentially we could solve the problem by finding the hundreds digit of <math>11^{2011}</math>. Powers of <math>11</math> are special because they can be represented by the Pascal's Triangle. Drawing the triangle, there is a theorem that states the powers of <math>11</math> can be found by reading rows of the triangle and adding extra numbers up. [add source] For example, the sixth row of the triangle is <math>1, 5, 10, 10, 5,</math> and <math>1</math>. Adding all numbers from right to left, we get <math>161051</math>, which is also <math>11^5</math>. In other words, each number is <math>10^n</math> steps from the right side of the row. The hundreds digit is <math>0</math>. We can do the same for <math>11^2011</math>, but we only need to find the <math>3</math> digits from the right. Observing, every <math>3</math> number from the right is <math>1 + 2 + 3... + n</math>. So to find the third number from the right on the row of <math>11^{2011}</math>, <math>f(11^n) = 1 + 2 + 3... + (n-1)</math>, or <math>\frac{(2010 * 2011)}{2}</math>, or <math>2021055</math>. The last digit is five, but we must remember to add the number on the right of it, which, by observing other rows is obviously <math>2011</math>. We must carry the <math>1</math> in <math>2011</math>'s tens digit to the <math>5</math> in <math>2021055</math>'s unit digit to get <math>\boxed{\textbf{(D) } 6}</math>. The one at the very end of the row doesn't affect anything, so we can leave it alone.
+
Notice that the hundreds digit of <math>2011^{2011}</math> won't be affected by <math>2000</math>. Essentially we could solve the problem by finding the hundreds digit of <math>11^{2011}</math>. Powers of <math>11</math> are special because they can be represented by the Pascal's Triangle. Drawing the triangle, there is a theorem that states the powers of <math>11</math> can be found by reading rows of the triangle and adding extra numbers up. [add source] For example, the sixth row of the triangle is <math>1, 5, 10, 10, 5,</math> and <math>1</math>. Adding all numbers from right to left, we get <math>161051</math>, which is also <math>11^5</math>. In other words, each number is <math>10^n</math> steps from the right side of the row. The hundreds digit is <math>0</math>. We can do the same for <math>11^{2011}</math>, but we only need to find the <math>3</math> digits from the right. Observing, every <math>3</math> number from the right is <math>1 + 2 + 3... + n</math>. So to find the third number from the right on the row of <math>11^{2011}</math>, <math>f(11^n) = 1 + 2 + 3... + (n-1)</math>, or <math>\frac{(2010 * 2011)}{2}</math>, or <math>2021055</math>. The last digit is five, but we must remember to add the number on the right of it, which, by observing other rows is obviously <math>2011</math>. We must carry the <math>1</math> in <math>2011</math>'s tens digit to the <math>5</math> in <math>2021055</math>'s unit digit to get <math>\boxed{\textbf{(D) } 6}</math>. The one at the very end of the row doesn't affect anything, so we can leave it alone.
  
  

Revision as of 18:56, 6 January 2020

Problem

What is the hundreds digit of $2011^{2011}?$

$\textbf{(A) } 1 \qquad \textbf{(B) } 4 \qquad \textbf{(C) }5 \qquad \textbf{(D) } 6 \qquad \textbf{(E) } 9$

Solution 1

Since $2011 \equiv 11 \pmod{1000},$ we know that $2011^{2011} \equiv 11^{2011} \pmod{1000}.$

To compute this, we use a clever application of the binomial theorem.

$\begin{aligned} 11^{2011} &= (1+10)^{2011} \\ &= 1 + \dbinom{2011}{1} \cdot 10 + \dbinom{2011}{2} \cdot 10^2 + \cdots \end{aligned}$

In all of the other terms, the power of $10$ is greater than $3$ and so is equivalent to $0$ modulo $1000,$ which means we can ignore it. We have:

$\begin{aligned}11^{2011} &\equiv 1 + 2011\cdot 10 + \dfrac{2011 \cdot 2010}{2} \cdot 100 \\ &\equiv 1+20110 + \dfrac{11\cdot 10}{2} \cdot 100\\ &= 1 + 20110 + 5500\\ &\equiv 1 + 110 + 500\\&=611 \pmod{1000} \end{aligned}$

Therefore, the hundreds digit is $\boxed{\textbf{(D) } 6}.$

Sidenote: By Euler's Totient Theorem, $a^{\phi (1000)} \equiv 1 \pmod{1000}$ for any $a$, so $a^{400} \equiv a \pmod{1000}$ and $11^{2011} \equiv 11^{11} \pmod{1000}$. We can then proceed using the clever application of the Binomial Theorem.

Solution 2

We need to compute $2011^{2011} \pmod{1000}.$ By the Chinese Remainder Theorem, it suffices to compute $2011^{2011} \pmod{8}$ and $2011^{2011} \pmod{125}.$

In modulo $8,$ we have $2011^4 \equiv 1 \pmod{8}$ by Euler's Theorem, and also $2011 \equiv 3 \pmod{8},$ so we have \[2011^{2011} = (2011^4)^{502} \cdot 2011^3 \equiv 1^{502} \cdot 3^3 \equiv 3 \pmod{8}.\]

In modulo $125,$ we have $2011^{100} \equiv 1 \pmod{125}$ by Euler's Theorem, and also $2011 \equiv 11 \pmod{125}.$ Therefore, we have $\begin{aligned} 2011^{2011} &= (2011^{100})^{20} \cdot 2011^{11} \\ &\equiv 1^{20} \cdot 11^{11} \\ &= 121^5 \cdot 11 \\ &= (-4)^5 \cdot 11 = -1024 \cdot 11 \\ &\equiv -24 \cdot 11 = -264 \\ &\equiv 111 \pmod{125}. \end{aligned}$

After finding the solution $2011^{2011} \equiv 611 \pmod{1000},$ we conclude it is the only one by the Chinese Remainder Theorem. Thus, the hundreds digit is $\boxed{\textbf{(D) } 6}.$

Solution 3

Notice that the hundreds digit of $2011^{2011}$ won't be affected by $2000$. Essentially we could solve the problem by finding the hundreds digit of $11^{2011}$. Powers of $11$ are special because they can be represented by the Pascal's Triangle. Drawing the triangle, there is a theorem that states the powers of $11$ can be found by reading rows of the triangle and adding extra numbers up. [add source] For example, the sixth row of the triangle is $1, 5, 10, 10, 5,$ and $1$. Adding all numbers from right to left, we get $161051$, which is also $11^5$. In other words, each number is $10^n$ steps from the right side of the row. The hundreds digit is $0$. We can do the same for $11^{2011}$, but we only need to find the $3$ digits from the right. Observing, every $3$ number from the right is $1 + 2 + 3... + n$. So to find the third number from the right on the row of $11^{2011}$, $f(11^n) = 1 + 2 + 3... + (n-1)$, or $\frac{(2010 * 2011)}{2}$, or $2021055$. The last digit is five, but we must remember to add the number on the right of it, which, by observing other rows is obviously $2011$. We must carry the $1$ in $2011$'s tens digit to the $5$ in $2021055$'s unit digit to get $\boxed{\textbf{(D) } 6}$. The one at the very end of the row doesn't affect anything, so we can leave it alone.


-jackshi2006


See Also

2011 AMC 10B (Problems • Answer Key • Resources)
Preceded by
Problem 22 		Followed by
Problem 24
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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