Because a circle has so much symmetry, there are many squares
whose four vertices all lie on the circle. For example, the points , , , and are the vertices of a square and
all lie on the circle with equation . As well, the four points ,
,
,
and
are the vertices of a square and all lie on the circle.
Note: In general, if we start with a point and rotate repeatedly by counterclockwise about the
origin, the four points obtained are always the vertices of a square.
Starting with the point with coordinates , the coordinates of the point
obtained by a rotation of
counterclockwise about the origin are . If we rotate counterclockwise two more
times, we get the points with coordinates and . Thus, for any real numbers and , the points with coordinates , , , and are the vertices of a square. This
will be useful in later parts. In addition, if is a point on the circle with
equation , then . Then since , the four
points , , , and , which are the vertices of a
square, all lie on the circle.
Observe that the four points , , , and are on the ellipse with equation
, as shown in the illustration below.
Because and , if is a point on the ellipse, then
and are both on the ellipse.
Therefore, the ellipse has reflective symmetry in the -axis and the -axis, which is also evident from the
diagram. It is reasonable to expect there to be a square inscribed in
the ellipse that also has reflective symmetry in the two axes. The
vertices of such a square should be of the form , , , and for some real number .
Notice that the point is
on the line with equation , so
assuming such a square exists, we should be able to find one of its
vertices by solving the system of equations
Substituting the first equation into the second and finding a common
denominator gives which rearranges to give
Since and are positive, , and so we get that
. We
therefore expect that the four points are all on the ellipse and are the vertices of a
square. As discussed in the solution to part (i), these points are of
the form , , , and , so they are the vertices of a
square. It is an exercise to verify that they are all on the
ellipse.
Below is a picture of the ellipse with a square inscribed.
Follow up question: If , then the ellipse is a circle and there are infinitely many
inscribed squares. If , is
the square in the solution above the only square inscribed in the
ellipse?
In this case, the loop is the regular hexagon pictured below.
Similar to the situation in part (ii), the loop has reflective
symmetry in both the and axes. Thus, we might again guess that
there is an inscribed square with reflective symmetry in both axes. If
such a square exists, then one of the vertices will have the form . Thus, we are looking for the
points (there are two of them) where the line with equation intersects the hexagon.
The line with equation must
intersect the part of the hexagon that is in the first quadrant. Notice
that
is above the line with equation , so the horizontal line segment in
the first quadrant will not intersect the line with equation . Therefore, the intersection point we
seek is on the line segment joining to , which
has equation . To find the point of intersection, we set
and solve to
get .
Recall that we hope the square has vertices at , , , and . You can check that with , these
four points lie on the given hexagon. Below is a picture of the hexagon
with the inscribed square.
Follow up question: There are two other squares
inscribed in the hexagon. They arise from rotating the square in the
solution by clockwise or
counterclockwise about
the origin. Are these the only possible squares?
The graphs of the relations and are pictured on the same axes below.
The loop, which is the boundary of the region enclosed by the two
graphs, is bold.
One approach is to try to find a square with two adjacent vertices on
the line with equation and the
other two vertices on the parabola. Why should we expect such a square
even exists? Imagine drawing a line that is parallel to the line with
equation , lies above the line
with equation , and intersects
the parabola twice. By drawing lines with slope through these two points of
intersection, we get a rectangle with two vertices on the parabola and
two vertices on the line. The diagrams below show such configurations
with the rectangle shaded in each case.
If the points of intersection of with the parabola are far apart, as in
the left diagram, then the two sides of the rectangle that are parallel
to the line are longer than the
other two sides. If the points of intersection are close together, as in
the right diagram, then the two sides of the rectangle that are parallel
to the line are shorter than
the other two sides. It seems reasonable to guess that somewhere in
between those two extremes is a happy medium where the rectangle is a
square. Formally, this kind of argument is an application of something
called the Intermediate Value Theorem.
Let’s use this thinking to find the coordinates of such a square. Let
be the line with equation where is the positive real number that will
cause the rectangle to be a square. The vertices of the square we hope
for are the points of intersection of with the parabola. To find these
points, we set equal to
and rearrange to get. where the last line is
obtained by multiplying the second-last line by .
We can now use the quadratic formula to get
Note: Since we are assuming that the line has two points of intersection with
the parabola, this quadratic equation must have two distinct real
solutions. This means we must have and so . Revisiting the diagrams, it should be clear
that there is an upper bound on the values of that can result in the line
intersecting the parabola. The diagrams also indicate that .
Therefore, the -coordinates of
the points of intersection are and .
The difference between the -coordinates of these points is and since the points
are on a line of slope , the
distance between the -coordinates
is also . Therefore,
the distance between the two points is
We will leave it as an exercise to show that the perpendicular
distance between and the line
with equation is equal to . This means our
square has sides of length and . A square has four
equal sides, so this implies the equation .
Squaring both sides, we have which can be rearranged to get . Factoring gives , so the possible values of
are and .
Since we need to be positive,
we conclude that , so the line
has equation . The -coordinates of the points of
intersection of with the parabola
were computed earlier. Substituting into these expressions, we get -coordinates of
and . These points lie on the line with equation , so the coordinates of two of the
vertices of the square are and
.
To find the other two vertices, we can find the equations of the
lines of slope through each of
these points, then find their intersection points with the line with
equation . We will not include
the calculations here, but the resulting points are
and .
Indeed, it is not difficult to check that the points are the vertices of a
square. Below is a plot of the loop along with the square with these
four points as its vertices.
The two parabolas are pictured below. The loop is bold.
By the discussion at the end of the solution to part (i), the four
points
are the vertices of a square. Furthermore, you can check that these four
points all lie on the loop.
For the rest of the solution to this part, we will show how one might
find these four points. The task is a bit trickier here because, unlike
in earlier parts, it is not easy to guess what the slope of the sides of
the square should be. However, there is still some symmetry that we can
use. Specifically, the loop has rotational symmetry about the
origin because the two parabolas are each the result of rotating the
other about the origin.
This is not difficult to believe from the diagram, but it can also be
verified algebraically.
This symmetry suggests two things. The first is that we should look
for a square that has
rotational symmetry about the origin. The second is that each of the two
parabolas should contain two vertices of the square.
A square that has
rotational symmetry about the origin must also have rotational symmetry about the
origin (you might want to take some time to convince yourself of this).
Again by the discussion from part (i), the vertices of the square should
be at the points with coordinates , , , and where and are some real numbers.
The parabola with equation should
contain two vertices of the square with the property that one of these
vertices is the result of rotating the other vertex about the origin. Thus, for
some and , this parabola should contain the
points and . This implies the following two
equations in and :
The second equation gives an expression for in terms of , so we can substitute this expression
into the first equation and simplify as follows: In general, we
should not expect to easily find a closed expression for the root of a
quartic polynomial. However, this question was designed to work out
nicely. After some checking, you might notice that is a root of this equation. Indeed,
. We
could factor to get but is the value we seek, so the other
roots of the quartic are not of any use in this solution (though you
might want to think about whether they have any "geometric" meaning).
With , the equation implies , so one of the points is
.
This is the first of the four points listed at the start of the solution
to this part. Rotating the point by , , and about the origin gives the
other three points. You can check that each of the two parabolas
contains two of these points. Below is a diagram of the loop with the
square included.