Difference between revisions of "2020 AMC 12A Problems/Problem 12"
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<math>\textbf{(A) } 10 \qquad \textbf{(B) } 15 \qquad \textbf{(C) } 20 \qquad \textbf{(D) } 25 \qquad \textbf{(E) } 30</math> | <math>\textbf{(A) } 10 \qquad \textbf{(B) } 15 \qquad \textbf{(C) } 20 \qquad \textbf{(D) } 25 \qquad \textbf{(E) } 30</math> | ||
− | == Solution == | + | == Solution 1== |
+ | [[File:AMC 12A 2020 Question-12 Solution-1.png|500px|right]] | ||
The slope of the line is <math>\frac{3}{5}</math>. We must transform it by <math>45^{\circ}</math>. | The slope of the line is <math>\frac{3}{5}</math>. We must transform it by <math>45^{\circ}</math>. | ||
− | <math>45^{\circ}</math> creates an isosceles right triangle since the sum of the angles of the triangle must be <math>180^{\circ}</math> and one angle is <math>90^{\circ}</math> | + | <math>45^{\circ}</math> creates an isosceles right triangle, since the sum of the angles of the triangle must be <math>180^{\circ}</math> and one angle is <math>90^{\circ}</math>. This means the last leg angle must also be <math>45^{\circ}</math>. |
− | In the isosceles right triangle, the two legs are congruent. We can | + | In the isosceles right triangle, the two legs are congruent. We can therefore construct an isosceles right triangle with a line of <math>\frac{3}{5}</math> slope on graph paper. That line with <math>\frac{3}{5}</math> slope starts at <math>(0,0)</math> and will go to <math>(5,3)</math>, the vector <math><5, 3 ></math>. |
Construct another line from <math>(0,0)</math> to <math>(3,-5)</math>, the vector <math><3,-5></math>. This is <math>\perp</math> and equal to the original line segment. The difference between the two vectors is <math><2,8></math>, which is the slope <math>4</math>, and that is the slope of line <math>k</math>. | Construct another line from <math>(0,0)</math> to <math>(3,-5)</math>, the vector <math><3,-5></math>. This is <math>\perp</math> and equal to the original line segment. The difference between the two vectors is <math><2,8></math>, which is the slope <math>4</math>, and that is the slope of line <math>k</math>. | ||
− | Furthermore, the equation <math>3x-5y+40=0</math> passes straight through <math>(20,20)</math> since <math>3(20)-5(20)+40=60-100+40=0</math>, which means that any rotations about <math>(20,20)</math> would contain <math>(20,20)</math>. We can create a line of slope <math>4</math> through <math>(20,20)</math>. The <math>x</math>-intercept is therefore <math>20-\frac{20}{4} = \boxed{\textbf{(B) } 15.}</math>~lopkiloinm | + | Furthermore, the equation <math>3x-5y+40=0</math> passes straight through <math>(20,20)</math> since <math>3(20)-5(20)+40=60-100+40=0</math>, which means that any rotations about <math>(20,20)</math> would contain <math>(20,20)</math>. We can create a line of slope <math>4</math> through <math>(20,20)</math>. The <math>x</math>-intercept is therefore <math>20-\frac{20}{4} = \boxed{\textbf{(B) } 15.}</math> ~lopkiloinm ~ShawnX (diagram) |
==Solution 2== | ==Solution 2== | ||
Line 48: | Line 49: | ||
~Silverdragon | ~Silverdragon | ||
+ | |||
+ | ==Solution 5 (Rotation Matrix)== | ||
+ | First note that the given line goes through <math>(20,20)</math> with a slope of <math>\frac{3}{5}</math>. This means that <math>(25,23)</math> is on the line. Now consider translating the graph so that <math>(20,20)</math> goes to the origin, then <math>(25,23)</math> becomes <math>(5,3)</math>. We now rotate the line <math>45^\circ</math> about the origin using a rotation matrix. This maps <math>(5,3)</math> to | ||
+ | <cmath>\begin{bmatrix} \frac{\sqrt{2}}{2} & -\frac{\sqrt{2}}{2} \\ \frac{\sqrt{2}}{2} & \frac{\sqrt{2}}{2}\end{bmatrix}\begin{bmatrix} 5 \\ 3\end{bmatrix}=\begin{bmatrix}\sqrt{2} \\ 4\sqrt{2}\end{bmatrix}</cmath> | ||
+ | The line through the origin and <math>(\sqrt{2}, 4\sqrt{2})</math> has slope <math>4</math>. Translating this line so that the origin is mapped to <math>(20,20)</math>, we find that the equation for the new line is <math>4x-60</math>, meaning that the <math>x</math>-intercept is <math>x=\frac{60}{4}=\boxed{\textbf{(B) }15}</math>. | ||
+ | |||
+ | ==Solution 6 (Angle Bisector)== | ||
+ | Note <math>(20,20)</math> is on the line. Construct the perpendicular line <math>5x+3y-160=0</math>. This creates a right triangle that intersects the x-axis at <math>(\frac{-40}{3},0)</math> and <math>(32,0)</math> a distance of <math>136/3</math> apart. The <math>45^\circ</math> transformation will bisect the right angle. | ||
+ | The angle bisector theorem tells us the <math>136</math> will split in ratio to the lengths of the sides. | ||
+ | These are <math>\sqrt{12^2+20^2}</math> and <math>\sqrt{\frac{100}{3}^2 + 20^2} = 4\sqrt{34}</math> and <math>\frac{20}{3}\sqrt{34}</math>. Thus the x intercept will split the line from <math>\frac{-40}{3}</math> to <math>32</math> into a ratio of <math>5:3</math> making the x-intercept <math>15</math>. | ||
+ | |||
+ | ==Solution 7 (Complex Numbers)== | ||
+ | Converting to the complex plane, we can see that two numbers on the line are <math>8i</math> and <math>5+11i</math>. Translating <math>20+20i</math> to the origin, we get <math>8i-20-20i = -20-12i</math> and <math>5+11i-20-20i = -15-9i</math>. Multiplying each of them by <math>e^{\pi i/4}</math>, we get <math>-4\sqrt 2 - 16 \sqrt2 i</math> and <math>-3\sqrt 2 - 12 \sqrt 2 i</math>. This line has a slope of <math>4</math>. Now, back to the cartesian plane. We have a line passing through <math>(20, 20)</math> with slope <math>4</math> which gives the equation as <math>y = 4x-60</math> which implies the <math>x</math> coordinate of the <math>x</math> intercept is <math>60/4 = 15</math>. | ||
+ | |||
+ | ~rocketsri (minor error corrected by kn07) | ||
+ | ==Solution 8 (quick)== | ||
+ | A quick check tells us that <math>(20,20)</math> falls on the given line. Common sense tells us that if the slope of the original line is <math>1</math>, or 45 degrees from the horizontal, a 45 counter clockwise rotation will result in a vertical line, and the x-intercept will be <math>(20,0)</math>. | ||
+ | Thinking of a 45 degree counter clockwise rotation as a 90 degree counter clockwise rotation that is bisected will helps in visualizing this line of reasoning. | ||
+ | Therefore, it follows that if the original line is made steeper, then the x-intercept will move away from <math>(20,0)</math> to the right. If the original line is made lower, then the opposite will happen. Our given line has slope <math>3/5</math>, so the answer must be <math>A</math> or <math>B</math>. <math>A</math> can be eliminated because an x-intercept of <math>(10,0)</math> can only occur when the original line is horizontal. In conclusion, the answer must be <math>\boxed{\textbf{(B) }15}</math>. | ||
+ | |||
+ | ~jackshi2006 | ||
==See Also== | ==See Also== | ||
{{AMC12 box|year=2020|ab=A|num-b=11|num-a=13}} | {{AMC12 box|year=2020|ab=A|num-b=11|num-a=13}} | ||
+ | |||
+ | [[Category:Introductory Geometry Problems]] | ||
{{MAA Notice}} | {{MAA Notice}} |
Latest revision as of 20:27, 5 November 2023
Contents
Problem
Line in the coordinate plane has equation
. This line is rotated
counterclockwise about the point
to obtain line
. What is the
-coordinate of the
-intercept of line
Solution 1
The slope of the line is . We must transform it by
.
creates an isosceles right triangle, since the sum of the angles of the triangle must be
and one angle is
. This means the last leg angle must also be
.
In the isosceles right triangle, the two legs are congruent. We can therefore construct an isosceles right triangle with a line of slope on graph paper. That line with
slope starts at
and will go to
, the vector
.
Construct another line from to
, the vector
. This is
and equal to the original line segment. The difference between the two vectors is
, which is the slope
, and that is the slope of line
.
Furthermore, the equation passes straight through
since
, which means that any rotations about
would contain
. We can create a line of slope
through
. The
-intercept is therefore
~lopkiloinm ~ShawnX (diagram)
Solution 2
Since the slope of the line is , and the angle we are rotating around is x, then
Hence, the slope of the rotated line is . Since we know the line intersects the point
, then we know the line is
. Set
to find the x-intercept, and so
~Solution by IronicNinja
Solution 3
Let be
and
be
and
respectively. Since the slope of the line is
we know that
Segments
and
represent the before and after of rotating
by 45 counterclockwise. Thus,
and
by tangent addition formula. Since
is 5 and the sidelength of the square is 20 the answer is
Solution 4 (Cheap)
Using the protractor you brought, carefully graph the equation and rotate the given line counter-clockwise about the point
. Scaling everything down by a factor of 5 makes this process easier.
It should then become fairly obvious that the x intercept is (only use this as a last resort).
~Silverdragon
Solution 5 (Rotation Matrix)
First note that the given line goes through with a slope of
. This means that
is on the line. Now consider translating the graph so that
goes to the origin, then
becomes
. We now rotate the line
about the origin using a rotation matrix. This maps
to
The line through the origin and
has slope
. Translating this line so that the origin is mapped to
, we find that the equation for the new line is
, meaning that the
-intercept is
.
Solution 6 (Angle Bisector)
Note is on the line. Construct the perpendicular line
. This creates a right triangle that intersects the x-axis at
and
a distance of
apart. The
transformation will bisect the right angle.
The angle bisector theorem tells us the
will split in ratio to the lengths of the sides.
These are
and
and
. Thus the x intercept will split the line from
to
into a ratio of
making the x-intercept
.
Solution 7 (Complex Numbers)
Converting to the complex plane, we can see that two numbers on the line are and
. Translating
to the origin, we get
and
. Multiplying each of them by
, we get
and
. This line has a slope of
. Now, back to the cartesian plane. We have a line passing through
with slope
which gives the equation as
which implies the
coordinate of the
intercept is
.
~rocketsri (minor error corrected by kn07)
Solution 8 (quick)
A quick check tells us that falls on the given line. Common sense tells us that if the slope of the original line is
, or 45 degrees from the horizontal, a 45 counter clockwise rotation will result in a vertical line, and the x-intercept will be
.
Thinking of a 45 degree counter clockwise rotation as a 90 degree counter clockwise rotation that is bisected will helps in visualizing this line of reasoning.
Therefore, it follows that if the original line is made steeper, then the x-intercept will move away from
to the right. If the original line is made lower, then the opposite will happen. Our given line has slope
, so the answer must be
or
.
can be eliminated because an x-intercept of
can only occur when the original line is horizontal. In conclusion, the answer must be
.
~jackshi2006
See Also
2020 AMC 12A (Problems • Answer Key • Resources) | |
Preceded by Problem 11 |
Followed by Problem 13 |
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 12 Problems and Solutions |
The problems on this page are copyrighted by the Mathematical Association of America's American Mathematics Competitions.