Question 60·Hard·Nonlinear Equations in One Variable; Systems in Two Variables
The circle with equation
and the parabola with equation
are graphed in the -plane. At how many points do the graphs of these two equations intersect?
For systems involving a circle and a parabola, first recognize the shapes and any obvious shifts (like or ). If both equations share the same shift, define new variables to move the shared point to the origin; this usually turns the equations into a standard circle and a simple parabola. Then solve by substitution: express one variable from the parabola, plug into the circle, and reduce the resulting equation to a quadratic in if needed. Finally, translate how many valid values you get into how many actual intersection points by considering the possible positive and negative square roots.
Hints
Identify the graphs and key features
Notice that one equation is a circle and the other is a parabola. Where is the center of the circle, and where is the vertex of the parabola?
Use a shift to simplify
Because both equations involve and , try defining new variables like and to rewrite both equations in a simpler form.
Solve the system algebraically
After rewriting, substitute the expression for from the parabola into the circle equation. You will get an equation in one variable; reduce it to a quadratic in and think about how many real values that produces.
Desmos Guide
Enter the equations in shifted form (optional but clearer)
You can either type the original equations or use the shifted ones. For a clearer view around the shared center/vertex, enter:
u^2 + v^2 = 25v = u^2
in Desmos, treating u as x and v as y (Desmos will still label them as x and y).
Graph the original equations (alternative)
If you prefer to use the original variables, enter:
(x-4)^2 + (y+2)^2 = 25y = (x-4)^2 - 2
Adjust the viewing window so that the point is near the center of the screen and the circle is fully visible.
Count the intersection points
Use Desmos’s intersection tool (click on the graphs where they cross, or tap the gray dots that appear) to mark all intersection points between the circle and the parabola. Count how many distinct intersection points appear; that count tells you which answer choice to select.
Step-by-step Explanation
Understand the shapes and their positions
Identify each graph:
- Circle: has center and radius .
- Parabola: has vertex and opens upward.
So both graphs are related to the point , which suggests shifting the coordinate system so that point becomes the origin. This will simplify the equations.
Shift the coordinates to simplify the equations
Let
Then and .
Rewrite both equations:
- Circle: becomes
- Parabola: means
so in this is
Now we need the intersection points of the circle and the parabola in the -plane.
Set up and solve the equation for intersection
Substitute into the circle equation :
This rearranges to
Let (so ). Then the equation becomes a quadratic in :
Solve this quadratic using the quadratic formula:
One of these values of is positive and the other is negative. Remember that cannot be negative, so only the positive value of corresponds to real intersection points.
Translate back to the number of intersection points
We found exactly one positive value of . A single positive value gives two different real values:
Each value gives a corresponding value using , so there are two distinct intersection points.
Translating back to using and does not change how many intersection points there are, only their locations.
Therefore, the graphs intersect at exactly two points.