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Difference between revisions of "2021 WSMO Accuracy Round Problems/Problem 7"
Line 14: Line 14:
 
<math></math>
 
<math></math>
 
\begin{align*}
 
\begin{align*}
−
2^n+3^n\equiv&\text{ }5\pmod{10}\tag{for <math>n\equiv1,3\pmod{4}</math>}\\
+
2^n+3^n\equiv&\text{ }5\pmod{10}\tag{for <math>n\equiv1,3\pmod{4}}\\
−
   2^n+3^n\equiv&\text{ }3\pmod{10}\tag{for <math>n\equiv2\pmod{4}</math>}\\
+
   2^n+3^n\equiv&\text{ }3\pmod{10}\tag{for </math>n\equiv2\pmod{4}<math>}\\
−
2^n+3^n\equiv&\text{ }7\pmod{10}\tag{for <math>n\equiv0\pmod{4}</math>}\\   
+
2^n+3^n\equiv&\text{ }7\pmod{10}\tag{for </math>n\equiv0\pmod{4}<math>}\\   
 
\end{align*}<cmath>
 
\end{align*}<cmath>
 
This means that  
 
This means that  
Line 27: Line 27:
 
     &=6500+18000+650\\
 
     &=6500+18000+650\\
 
     &=\boxed{25150}\\
 
     &=\boxed{25150}\\
−
\end{align*}<math></math>
+
\end{align*}</math><math>
 
~pinkpig
 
~pinkpig
  
 
==Solution 2==
 
==Solution 2==
−
We will begin by examining <math>2^i\text{ (mod }10\text{)}</math> and <math>3^i\text{ (mod }10\text{)}</math>:
+
We will begin by examining </math>2^i\text{ (mod }10\text{)}<math> and </math>3^i\text{ (mod }10\text{)}<math>:
  
 
<cmath>2^1 \equiv 2\text{ (mod }10\text{)}</cmath>
 
<cmath>2^1 \equiv 2\text{ (mod }10\text{)}</cmath>
Line 59: Line 59:
  
  
−
Note that <math>100+96+...+4 = 4(25+24+...+1) = 4(\frac{25 \cdot 26}{2}) = 1300</math>:
+
Note that </math>100+96+...+4 = 4(25+24+...+1) = 4(\frac{25 \cdot 26}{2}) = 1300$:
  
 
<cmath>=1300r_1 + (1300-25)r_2 + (1300-50)r_3 + (1300-75)r_4</cmath>
 
<cmath>=1300r_1 + (1300-25)r_2 + (1300-50)r_3 + (1300-75)r_4</cmath>

Revision as of 10:22, 11 July 2022

Problem

Find the value of $\sum_{n=1}^{100}\left(\sum_{i=1}^{n}r_i\right),$ where $r_i$ is the remainder when $2^i+3^i$ is divided by 10.

Solution 1

From Fermat's Little Theorem, we find that $a^n\pmod{10}$ is periodic in cycles of 4. This means that $a^n\equiv a^{n+4}\pmod{10}$ for all $a,n.$ Now, we will compute the first 4 values of $2^n+3^n\pmod{10}.$ \begin{align*} 2^1+3^1\equiv2+3\equiv&\text{ }5\pmod{10}\\ 2^2+3^2\equiv4+9\equiv13\equiv&\text{ }3\pmod{10}\\ 2^3+3^3\equiv8+27\equiv35\equiv&\text{ }5\pmod{10}\\ 2^4+3^4\equiv16+81\equiv97\equiv&\text{ }7\pmod{10}\\ \end{align*} Thus, $$ (Error compiling LaTeX. Unknown error_msg) \begin{align*} 2^n+3^n\equiv&\text{ }5\pmod{10}\tag{for $n\equiv1,3\pmod{4}}\\ 

 	2^n+3^n\equiv&\text{ }3\pmod{10}\tag{for$ (Error compiling LaTeX. Unknown error_msg)n\equiv2\pmod{4}$}\\

2^n+3^n\equiv&\text{ }7\pmod{10}\tag{for$ (Error compiling LaTeX. Unknown error_msg)n\equiv0\pmod{4}$}\\ \end{align*}<cmath> This means that </cmath> \begin{align*} \sum_{n=1}^{100}\left(\sum_{i=1}^{n}r_i\right)&=\sum_{n=4a,1\leq a\leq25}(20a)+\sum_{n=4a+1,0\leq a\leq24}(20a+5)+\sum_{n=4a+2,0\leq a\leq24}(20a+8)+\sum_{n=4a+3,0\leq a\leq24}(20a+13)\\ 

   &=\frac{25\cdot26}{2}\cdot20+\left(\frac{24\cdot25}{2}\cdot20+25\cdot5\right)+\left(\frac{24\cdot25}{2}\cdot20+25\cdot8\right)+\left(\frac{24\cdot25}{2}\cdot20+25\cdot13\right)\\
   &=325\cdot20+300\cdot20\cdot3+25\cdot(5+8+13)\\
   &=6500+6000\cdot3+25\cdot26\\
   &=6500+18000+650\\
   &=\boxed{25150}\\

\end{align*}$ (Error compiling LaTeX. Unknown error_msg)$~pinkpig

==Solution 2== We will begin by examining$ (Error compiling LaTeX. Unknown error_msg)2^i\text{ (mod }10\text{)}$and$3^i\text{ (mod }10\text{)}$:

<cmath>2^1 \equiv 2\text{ (mod }10\text{)}</cmath> <cmath>2^2 \equiv 4\text{ (mod }10\text{)}</cmath> <cmath>2^3 \equiv 8\text{ (mod }10\text{)}</cmath> <cmath>2^4 \equiv 6\text{ (mod }10\text{)}</cmath> <cmath>...</cmath> and <cmath>3^1 \equiv 3\text{ (mod }10\text{)}</cmath> <cmath>3^2 \equiv 9\text{ (mod }10\text{)}</cmath> <cmath>3^3 \equiv 7\text{ (mod }10\text{)}</cmath> <cmath>3^4 \equiv 1\text{ (mod }10\text{)}</cmath> <cmath>...</cmath>

From this, we can note that:

<cmath>r_i = 5 \text{, } i \equiv 1 \text{ (mod }4\text{)}</cmath> <cmath>r_i = 3 \text{, } i \equiv 2 \text{ (mod }4\text{)}</cmath> <cmath>r_i = 5 \text{, } i \equiv 3 \text{ (mod }4\text{)}</cmath> <cmath>r_i = 7 \text{, } i \equiv 0 \text{ (mod }4\text{)}</cmath>

We can simplify our sum as follows:

<cmath>\sum_{n=1}^{100}\left(\sum_{i=1}^{n}r_i\right)</cmath> <cmath>=100r_1 + 99r_2 + 98r_3 + ... + r_{100}</cmath> <cmath>=(100+96+...+4)r_1 + (99+95+...+3)r_2 + (98+94+...+2)r_3 + (97+93+...+1)r_4</cmath>


Note that$ (Error compiling LaTeX. Unknown error_msg)100+96+...+4 = 4(25+24+...+1) = 4(\frac{25 \cdot 26}{2}) = 1300$:

\[=1300r_1 + (1300-25)r_2 + (1300-50)r_3 + (1300-75)r_4\] \[=5(1300) + 3(1275) + 5(1250) + 7(1225)\] \[=\boxed{25150}\]

~BigKahuna227

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