Question 59·Medium·Nonlinear Equations in One Variable; Systems in Two Variables
When the equations above are graphed in the -plane, what are the coordinates of the points of intersection of the two graphs?
For systems involving a line and a circle (or any nonlinear equation), use substitution whenever one equation is already solved for a variable, like . Substitute that expression into the other equation to get a single-variable equation, simplify carefully (expand squares, combine like terms), and solve the resulting quadratic. Then plug each solution back into the simpler equation to find matching coordinates, and finally verify that your points satisfy both equations before matching them to the answer choices.
Hints
Identify what each equation represents
Think about what shape represents, and what shape represents in the -plane.
Use substitution
Because one equation is already solved for , try substituting into the other equation so you can solve for .
Solve the resulting equation carefully
After you substitute, you will get a quadratic equation in . Expand , simplify, and factor the quadratic to find the two -values.
Find the matching y-values
Once you have the possible -values, plug each back into to get the corresponding -values, then compare your points to the answer choices.
Desmos Guide
Graph the circle
In Desmos, type x^2 + y^2 = 25 to graph the circle centered at the origin with radius 5.
Graph the line
On a new line in Desmos, type y = x + 1 to graph the line.
Locate the intersection points
Click or tap on each point where the line and the circle cross. Desmos will display the coordinates of these intersection points; compare those coordinates to the answer choices.
Step-by-step Explanation
Recognize the shapes of the graphs
The first equation is a circle centered at the origin with radius (because ).
The second equation is a straight line with slope and -intercept .
Their intersection points are the points that satisfy both equations at the same time.
Use substitution to combine the equations
Since , substitute for in the circle equation:
Now you have an equation in terms of only.
Expand and simplify to get a quadratic equation
Expand and simplify:
Subtract from both sides:
Divide everything by to simplify:
This is a quadratic equation in .
Solve the quadratic equation for x
Factor the quadratic :
Set each factor equal to zero:
- gives .
- gives .
So the possible -values of the intersection points are and .
Find the corresponding y-values and state the intersection points
Use to find for each :
- If , then , giving the point .
- If , then , giving the point .
So, the two points of intersection are and , which matches choice A.