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Difference between revisions of "2023 AMC 8 Problems/Problem 4"

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(→‎Video Solution by Math-X (Smart and Simple))
 
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The numbers from 1 to 49 are arranged in a spiral pattern on a square grid, beginning at the center. The first few numbers have been entered into the grid below. Consider the four numbers that will appear in the shaded squares, on the same diagonal as the number 7. How many of these four numbers are prime?
+
==Problem==
  
 +
The numbers from <math>1</math> to <math>49</math> are arranged in a spiral pattern on a square grid, beginning at the center. The first few numbers have been entered into the grid below. Consider the four numbers that will appear in the shaded squares, on the same diagonal as the number <math>7.</math> How many of these four numbers are prime?
 +
<asy>
 +
/* Made by MRENTHUSIASM */
 +
size(175);
 +
 +
void ds(pair p) {
 +
filldraw((0.5,0.5)+p--(-0.5,0.5)+p--(-0.5,-0.5)+p--(0.5,-0.5)+p--cycle,mediumgrey);
 +
}
 +
 +
ds((0.5,4.5));
 +
ds((1.5,3.5));
 +
ds((3.5,1.5));
 +
ds((4.5,0.5));
 +
 +
add(grid(7,7,grey+linewidth(1.25)));
 +
 +
int adj = 1;
 +
int curUp = 2;
 +
int curLeft = 4;
 +
int curDown = 6;
 +
 +
label("$1$",(3.5,3.5));
 +
 +
for (int len = 3; len<=3; len+=2)
 +
{
 +
for (int i=1; i<=len-1; ++i)
 +
  {
 +
label("$"+string(curUp)+"$",(3.5+adj,3.5-adj+i));
 +
    label("$"+string(curLeft)+"$",(3.5+adj-i,3.5+adj));
 +
    label("$"+string(curDown)+"$",(3.5-adj,3.5+adj-i));
 +
    ++curDown;
 +
    ++curLeft;
 +
    ++curUp;
 +
}
 +
++adj;
 +
    curUp = len^2 + 1;
 +
    curLeft = len^2 + len + 2;
 +
    curDown = len^2 + 2*len + 3;
 +
}
 +
 +
draw((4,4)--(3,4)--(3,3)--(5,3)--(5,5)--(2,5)--(2,2)--(6,2)--(6,6)--(1,6)--(1,1)--(7,1)--(7,7)--(0,7)--(0,0)--(7,0),linewidth(2));
 +
</asy>
 
<math>\textbf{(A)}\ 0 \qquad \textbf{(B)}\ 1 \qquad \textbf{(C)}\ 2 \qquad \textbf{(D)}\ 3 \qquad \textbf{(E)}\ 4</math>
 
<math>\textbf{(A)}\ 0 \qquad \textbf{(B)}\ 1 \qquad \textbf{(C)}\ 2 \qquad \textbf{(D)}\ 3 \qquad \textbf{(E)}\ 4</math>
  
 
==Solution 1==
 
==Solution 1==
  
−
First, we fill out the entire grid. We find that the four numbers are <math>39,19,23,47</math>. The numbers <math>19,23,</math> and <math>47</math> are prime, so there are <math>\boxed{\textbf{(D) }3}</math> prime numbers.
+
We fill out the grid, as shown below:
 +
<asy>
 +
/* Made by MRENTHUSIASM */
 +
size(175);
 +
 
 +
void ds(pair p) {
 +
filldraw((0.5,0.5)+p--(-0.5,0.5)+p--(-0.5,-0.5)+p--(0.5,-0.5)+p--cycle,mediumgrey);
 +
}
  
−
~MathFun1000
+
ds((0.5,4.5));
 +
ds((1.5,3.5));
 +
ds((3.5,1.5));
 +
ds((4.5,0.5));
 +
 
 +
add(grid(7,7,grey+linewidth(1.25)));
 +
 
 +
int adj = 1;
 +
int curUp = 2;
 +
int curLeft = 4;
 +
int curDown = 6;
 +
int curRight = 8;
 +
 
 +
label("$1$",(3.5,3.5));
 +
 
 +
for (int len = 3; len<=7; len+=2)
 +
{
 +
for (int i=1; i<=len-1; ++i)
 +
  {
 +
label("$"+string(curUp)+"$",(3.5+adj,3.5-adj+i));
 +
    label("$"+string(curLeft)+"$",(3.5+adj-i,3.5+adj));
 +
    label("$"+string(curDown)+"$",(3.5-adj,3.5+adj-i));
 +
    label("$"+string(curRight)+"$",(3.5-adj+i,3.5-adj));
 +
    ++curDown;
 +
    ++curLeft;
 +
    ++curUp;
 +
    ++curRight;
 +
}
 +
++adj;
 +
    curUp = len^2 + 1;
 +
    curLeft = len^2 + len + 2;
 +
    curDown = len^2 + 2*len + 3;
 +
    curRight = len^2 + 3*len + 4;
 +
}
 +
 
 +
draw((4,4)--(3,4)--(3,3)--(5,3)--(5,5)--(2,5)--(2,2)--(6,2)--(6,6)--(1,6)--(1,1)--(7,1)--(7,7)--(0,7)--(0,0)--(7,0),linewidth(2));
 +
</asy>
 +
From the four numbers that appear in the shaded squares, <math>\boxed{\textbf{(D)}\ 3}</math> of them are prime: <math>19,23,</math> and <math>47.</math>
 +
 
 +
~MathFun1000, MRENTHUSIASM
  
 
==Solution 2==
 
==Solution 2==
 +
Note that given time constraint, it's better to only count from perfect squares (in pink), as shown below:
 +
<asy>
 +
/* Grid Made by MRENTHUSIASM */
 +
/* Squares pattern solution input by TheMathGuyd */
 +
size(175);
 +
 +
void ds(pair p) {
 +
filldraw((0.5,0.5)+p--(-0.5,0.5)+p--(-0.5,-0.5)+p--(0.5,-0.5)+p--cycle,mediumgrey);
 +
}
 +
 +
void ps(pair p) {
 +
filldraw((0.5,0.5)+p--(-0.5,0.5)+p--(-0.5,-0.5)+p--(0.5,-0.5)+p--cycle,pink+opacity(0.3));
 +
}
 +
real ts = 0.5;
 +
 +
ds((0.5,4.5));label("$39$",(0.5,4.5));
 +
ds((1.5,3.5));label("$19$",(1.5,3.5));
 +
ds((3.5,1.5));label("$23$",(3.5,1.5));
 +
ds((4.5,0.5));label("$47$",(4.5,0.5));
 +
 +
ps((3.5,3.5));label("$1$",(3.5,3.5));
 +
ps((4.5,2.5));label("$9$",(4.5,2.5));
 +
ps((5.5,1.5));label("$25$",(5.5,1.5));
 +
ps((6.5,0.5));label("$49$",(6.5,0.5));
 +
ps((3.5,4.5));label("$4$",(3.5,4.5));
 +
ps((2.5,5.5));label("$16$",(2.5,5.5));
 +
ps((1.5,6.5));label("$36$",(1.5,6.5));
 +
label(scale(ts)*"$\leftarrow$",(1,6),NE);
 +
label(scale(ts)*"$+1$",(1,6),NW);
 +
label(scale(ts)*"$\downarrow$",(1,6),SW);
 +
label(scale(ts)*"$+2$",(1,5),NW);
 +
label(scale(ts)*"$\downarrow$",(1,5),SW);
 +
label(scale(ts)*"$+3$",(1,4),NW);
 +
label(scale(ts)*"$+1$",(2,5),NW);
 +
label(scale(ts)*"$\downarrow$",(2,5),SW);
 +
label(scale(ts)*"$+2$",(2,4),NW);
 +
label(scale(ts)*"$\downarrow$",(2,4),SW);
 +
label(scale(ts)*"$+3$",(2,3),NW);
 +
 +
label(scale(ts)*"$\leftarrow$",(5,1),NE);
 +
label(scale(ts)*"$-1$",(5,1),NW);
 +
label(scale(ts)*"$\leftarrow$",(4,1),NE);
 +
label(scale(ts)*"$-2$",(4,1),NW);
 +
label(scale(ts)*"$\leftarrow$",(6,0),NE);
 +
label(scale(ts)*"$-1$",(6,0),NW);
 +
label(scale(ts)*"$\leftarrow$",(5,0),NE);
 +
label(scale(ts)*"$-2$",(5,0),NW);
 +
 +
add(grid(7,7,grey+linewidth(1.25))); //USES OLYMPIAD.ASY
 +
 +
draw((4,4)--(3,4)--(3,3)--(5,3)--(5,5)--(2,5)--(2,2)--(6,2)--(6,6)--(1,6)--(1,1)--(7,1)--(7,7)--(0,7)--(0,0)--(7,0),linewidth(2));
 +
</asy>
 +
From the four numbers that appear in the shaded squares, <math>\boxed{\textbf{(D)}\ 3}</math> of them are prime: <math>19,23,</math> and <math>47.</math>
 +
 +
~TheMathGuyd
 +
 +
==Simple, Intuitive Solution by MathTalks_Now==
 +
* *Different Solution not shown before!*
 +
 +
https://studio.youtube.com/video/PMOeiGLkDH0/edit
 +
 +
==Video Solution (How to Creatively THINK!!!)==
 +
https://youtu.be/7gwhzjySKl0
 +
 +
~Education the Study of everything
 +
 +
==Video Solution by Math-X (Smart and Simple)==
 +
https://youtu.be/Ku_c1YHnLt0?si=cc_Ii2j2pmT6wOuZ&t=412
 +
 +
~Math-X
 +
 +
==Video Solution by Magic Square==
 +
https://youtu.be/-N46BeEKaCQ?t=5392
 +
 +
==Video Solution by SpreadTheMathLove==
 +
https://www.youtube.com/watch?v=EcrktBc8zrM
 +
==Video Solution by Interstigation==
 +
https://youtu.be/DBqko2xATxs&t=233
 +
 +
==Video Solution by WhyMath==
 +
https://youtu.be/1qwfPJDNYGc
 +
 +
~savannahsolver
 +
 +
==Video Solution by harungurcan==
 +
https://www.youtube.com/watch?v=35BW7bsm_Cg&t=402s
 +
 +
~harungurcan
  
−
Fill out the entire grid to count that there are <math>\boxed{\text{(D)}3}</math>
+
==See Also==
−
-apex304
+
{{AMC8 box|year=2023|num-b=3|num-a=5}}
 +
{{MAA Notice}}

Latest revision as of 15:01, 25 January 2024

Problem

The numbers from $1$ to $49$ are arranged in a spiral pattern on a square grid, beginning at the center. The first few numbers have been entered into the grid below. Consider the four numbers that will appear in the shaded squares, on the same diagonal as the number $7.$ How many of these four numbers are prime? [asy] /* Made by MRENTHUSIASM */ size(175); void ds(pair p) { filldraw((0.5,0.5)+p--(-0.5,0.5)+p--(-0.5,-0.5)+p--(0.5,-0.5)+p--cycle,mediumgrey); } ds((0.5,4.5)); ds((1.5,3.5)); ds((3.5,1.5)); ds((4.5,0.5)); add(grid(7,7,grey+linewidth(1.25))); int adj = 1; int curUp = 2; int curLeft = 4; int curDown = 6; label("$1$",(3.5,3.5)); for (int len = 3; len<=3; len+=2) { for (int i=1; i<=len-1; ++i) { label("$"+string(curUp)+"$",(3.5+adj,3.5-adj+i)); label("$"+string(curLeft)+"$",(3.5+adj-i,3.5+adj)); label("$"+string(curDown)+"$",(3.5-adj,3.5+adj-i)); ++curDown; ++curLeft; ++curUp; } ++adj; curUp = len^2 + 1; curLeft = len^2 + len + 2; curDown = len^2 + 2*len + 3; } draw((4,4)--(3,4)--(3,3)--(5,3)--(5,5)--(2,5)--(2,2)--(6,2)--(6,6)--(1,6)--(1,1)--(7,1)--(7,7)--(0,7)--(0,0)--(7,0),linewidth(2)); [/asy] $\textbf{(A)}\ 0 \qquad \textbf{(B)}\ 1 \qquad \textbf{(C)}\ 2 \qquad \textbf{(D)}\ 3 \qquad \textbf{(E)}\ 4$

Solution 1

We fill out the grid, as shown below: [asy] /* Made by MRENTHUSIASM */ size(175); void ds(pair p) { filldraw((0.5,0.5)+p--(-0.5,0.5)+p--(-0.5,-0.5)+p--(0.5,-0.5)+p--cycle,mediumgrey); } ds((0.5,4.5)); ds((1.5,3.5)); ds((3.5,1.5)); ds((4.5,0.5)); add(grid(7,7,grey+linewidth(1.25))); int adj = 1; int curUp = 2; int curLeft = 4; int curDown = 6; int curRight = 8; label("$1$",(3.5,3.5)); for (int len = 3; len<=7; len+=2) { for (int i=1; i<=len-1; ++i) { label("$"+string(curUp)+"$",(3.5+adj,3.5-adj+i)); label("$"+string(curLeft)+"$",(3.5+adj-i,3.5+adj)); label("$"+string(curDown)+"$",(3.5-adj,3.5+adj-i)); label("$"+string(curRight)+"$",(3.5-adj+i,3.5-adj)); ++curDown; ++curLeft; ++curUp; ++curRight; } ++adj; curUp = len^2 + 1; curLeft = len^2 + len + 2; curDown = len^2 + 2*len + 3; curRight = len^2 + 3*len + 4; } draw((4,4)--(3,4)--(3,3)--(5,3)--(5,5)--(2,5)--(2,2)--(6,2)--(6,6)--(1,6)--(1,1)--(7,1)--(7,7)--(0,7)--(0,0)--(7,0),linewidth(2)); [/asy] From the four numbers that appear in the shaded squares, $\boxed{\textbf{(D)}\ 3}$ of them are prime: $19,23,$ and $47.$

~MathFun1000, MRENTHUSIASM

Solution 2

Note that given time constraint, it's better to only count from perfect squares (in pink), as shown below: [asy] /* Grid Made by MRENTHUSIASM */ /* Squares pattern solution input by TheMathGuyd */ size(175); void ds(pair p) { filldraw((0.5,0.5)+p--(-0.5,0.5)+p--(-0.5,-0.5)+p--(0.5,-0.5)+p--cycle,mediumgrey); } void ps(pair p) { filldraw((0.5,0.5)+p--(-0.5,0.5)+p--(-0.5,-0.5)+p--(0.5,-0.5)+p--cycle,pink+opacity(0.3)); } real ts = 0.5; ds((0.5,4.5));label("$39$",(0.5,4.5)); ds((1.5,3.5));label("$19$",(1.5,3.5)); ds((3.5,1.5));label("$23$",(3.5,1.5)); ds((4.5,0.5));label("$47$",(4.5,0.5)); ps((3.5,3.5));label("$1$",(3.5,3.5)); ps((4.5,2.5));label("$9$",(4.5,2.5)); ps((5.5,1.5));label("$25$",(5.5,1.5)); ps((6.5,0.5));label("$49$",(6.5,0.5)); ps((3.5,4.5));label("$4$",(3.5,4.5)); ps((2.5,5.5));label("$16$",(2.5,5.5)); ps((1.5,6.5));label("$36$",(1.5,6.5)); label(scale(ts)*"$\leftarrow$",(1,6),NE); label(scale(ts)*"$+1$",(1,6),NW); label(scale(ts)*"$\downarrow$",(1,6),SW); label(scale(ts)*"$+2$",(1,5),NW); label(scale(ts)*"$\downarrow$",(1,5),SW); label(scale(ts)*"$+3$",(1,4),NW); label(scale(ts)*"$+1$",(2,5),NW); label(scale(ts)*"$\downarrow$",(2,5),SW); label(scale(ts)*"$+2$",(2,4),NW); label(scale(ts)*"$\downarrow$",(2,4),SW); label(scale(ts)*"$+3$",(2,3),NW); label(scale(ts)*"$\leftarrow$",(5,1),NE); label(scale(ts)*"$-1$",(5,1),NW); label(scale(ts)*"$\leftarrow$",(4,1),NE); label(scale(ts)*"$-2$",(4,1),NW); label(scale(ts)*"$\leftarrow$",(6,0),NE); label(scale(ts)*"$-1$",(6,0),NW); label(scale(ts)*"$\leftarrow$",(5,0),NE); label(scale(ts)*"$-2$",(5,0),NW); add(grid(7,7,grey+linewidth(1.25))); //USES OLYMPIAD.ASY draw((4,4)--(3,4)--(3,3)--(5,3)--(5,5)--(2,5)--(2,2)--(6,2)--(6,6)--(1,6)--(1,1)--(7,1)--(7,7)--(0,7)--(0,0)--(7,0),linewidth(2)); [/asy] From the four numbers that appear in the shaded squares, $\boxed{\textbf{(D)}\ 3}$ of them are prime: $19,23,$ and $47.$

~TheMathGuyd

Simple, Intuitive Solution by MathTalks_Now

  • *Different Solution not shown before!*

https://studio.youtube.com/video/PMOeiGLkDH0/edit

Video Solution (How to Creatively THINK!!!)

https://youtu.be/7gwhzjySKl0

~Education the Study of everything

Video Solution by Math-X (Smart and Simple)

https://youtu.be/Ku_c1YHnLt0?si=cc_Ii2j2pmT6wOuZ&t=412

~Math-X

Video Solution by Magic Square

https://youtu.be/-N46BeEKaCQ?t=5392

Video Solution by SpreadTheMathLove

https://www.youtube.com/watch?v=EcrktBc8zrM

Video Solution by Interstigation

https://youtu.be/DBqko2xATxs&t=233

Video Solution by WhyMath

https://youtu.be/1qwfPJDNYGc

~savannahsolver

Video Solution by harungurcan

https://www.youtube.com/watch?v=35BW7bsm_Cg&t=402s

~harungurcan

See Also

2023 AMC 8 (Problems • Answer Key • Resources)
Preceded by
Problem 3 		Followed by
Problem 5
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 AJHSME/AMC 8 Problems and Solutions

The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions. AMC logo.png

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