Difference between revisions of "2019 AMC 8 Problems/Problem 6"

m (Problem 6)
(Solution 1)
Line 198: Line 198:
 
dot((8,8));
 
dot((8,8));
 
label("P",(4,4),NE);
 
label("P",(4,4),NE);
 +
draw((0,8)--(8,0));
 +
draw(8,8)--(0,0));
 
</asy>
 
</asy>
 
Lines of symmetry go through point P, and there are 8 directions the lines could go, and there are 4 dots at each direction. 4<math>*</math>8/80=<math>\boxed{\textbf{(C)}\ \frac{2}{5}}</math>.
 
Lines of symmetry go through point P, and there are 8 directions the lines could go, and there are 4 dots at each direction. 4<math>*</math>8/80=<math>\boxed{\textbf{(C)}\ \frac{2}{5}}</math>.

Revision as of 13:16, 22 November 2019

Problem 6

There are $81$ grid points (uniformly spaced) in the square shown in the diagram below, including the points on the edges. Point $P$ is in the center of the square. Given that point $Q$ is randomly chosen among the other $80$ points, what is the probability that the line $PQ$ is a line of symmetry for the square?

[asy] draw((0,0)--(0,8)); draw((0,8)--(8,8)); draw((8,8)--(8,0)); draw((8,0)--(0,0)); dot((0,0)); dot((0,1)); dot((0,2)); dot((0,3)); dot((0,4)); dot((0,5)); dot((0,6)); dot((0,7)); dot((0,8));  dot((1,0)); dot((1,1)); dot((1,2)); dot((1,3)); dot((1,4)); dot((1,5)); dot((1,6)); dot((1,7)); dot((1,8));  dot((2,0)); dot((2,1)); dot((2,2)); dot((2,3)); dot((2,4)); dot((2,5)); dot((2,6)); dot((2,7)); dot((2,8));  dot((3,0)); dot((3,1)); dot((3,2)); dot((3,3)); dot((3,4)); dot((3,5)); dot((3,6)); dot((3,7)); dot((3,8));  dot((4,0)); dot((4,1)); dot((4,2)); dot((4,3)); dot((4,4)); dot((4,5)); dot((4,6)); dot((4,7)); dot((4,8));  dot((5,0)); dot((5,1)); dot((5,2)); dot((5,3)); dot((5,4)); dot((5,5)); dot((5,6)); dot((5,7)); dot((5,8));  dot((6,0)); dot((6,1)); dot((6,2)); dot((6,3)); dot((6,4)); dot((6,5)); dot((6,6)); dot((6,7)); dot((6,8));  dot((7,0)); dot((7,1)); dot((7,2)); dot((7,3)); dot((7,4)); dot((7,5)); dot((7,6)); dot((7,7)); dot((7,8));  dot((8,0)); dot((8,1)); dot((8,2)); dot((8,3)); dot((8,4)); dot((8,5)); dot((8,6)); dot((8,7)); dot((8,8)); label("P",(4,4),NE); [/asy]

$\textbf{(A) }\frac{1}{5}\qquad\textbf{(B) }\frac{1}{4} \qquad\textbf{(C) }\frac{2}{5} \qquad\textbf{(D) }\frac{9}{20} \qquad\textbf{(E) }\frac{1}{2}$

Solution 1

draw((0,0)--(0,8));
draw((0,8)--(8,8));
draw((8,8)--(8,0));
draw((8,0)--(0,0));
dot((0,0));
dot((0,1));
dot((0,2));
dot((0,3));
dot((0,4));
dot((0,5));
dot((0,6));
dot((0,7));
dot((0,8));

dot((1,0));
dot((1,1));
dot((1,2));
dot((1,3));
dot((1,4));
dot((1,5));
dot((1,6));
dot((1,7));
dot((1,8));

dot((2,0));
dot((2,1));
dot((2,2));
dot((2,3));
dot((2,4));
dot((2,5));
dot((2,6));
dot((2,7));
dot((2,8));

dot((3,0));
dot((3,1));
dot((3,2));
dot((3,3));
dot((3,4));
dot((3,5));
dot((3,6));
dot((3,7));
dot((3,8));

dot((4,0));
dot((4,1));
dot((4,2));
dot((4,3));
dot((4,4));
dot((4,5));
dot((4,6));
dot((4,7));
dot((4,8));

dot((5,0));
dot((5,1));
dot((5,2));
dot((5,3));
dot((5,4));
dot((5,5));
dot((5,6));
dot((5,7));
dot((5,8));

dot((6,0));
dot((6,1));
dot((6,2));
dot((6,3));
dot((6,4));
dot((6,5));
dot((6,6));
dot((6,7));
dot((6,8));

dot((7,0));
dot((7,1));
dot((7,2));
dot((7,3));
dot((7,4));
dot((7,5));
dot((7,6));
dot((7,7));
dot((7,8));

dot((8,0));
dot((8,1));
dot((8,2));
dot((8,3));
dot((8,4));
dot((8,5));
dot((8,6));
dot((8,7));
dot((8,8));
label("P",(4,4),NE);
draw((0,8)--(8,0));
draw(8,8)--(0,0));
 (Error making remote request. Unknown error_msg)

Lines of symmetry go through point P, and there are 8 directions the lines could go, and there are 4 dots at each direction. 4$*$8/80=$\boxed{\textbf{(C)}\ \frac{2}{5}}$. ~heeeeeheeeeeee

See Also

2019 AMC 8 (ProblemsAnswer KeyResources)
Preceded by
Problem 5
Followed by
Problem 7
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

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