Question 225·Medium·Nonlinear Equations in One Variable; Systems in Two Variables
Consider the system of equations:
Which ordered pair (x, y) satisfies the system?
For systems with one linear equation and one nonlinear equation (like a circle), a fast algebraic method is to use identities or substitution to connect them. Here, using with the given and quickly yields , then you build a simple quadratic whose roots are and . On timed tests, also remember that with only four answer choices you can often quickly check each ordered pair by plugging into both equations and seeing which one satisfies them all.
Hints
Connect the two equations
You know and you know . How could you combine these to get more information about and together?
Use a square identity
Try squaring and use the identity to bring in the term from the second equation.
After finding
Once you know both and , think of and as the roots of a quadratic equation whose coefficients come from that sum and product.
Desmos Guide
Graph the line
In Desmos, enter the first equation as y = 5 - x to graph the line representing .
Graph the circle
Enter the second equation as x^2 + y^2 = 13; Desmos will display a circle centered at the origin with radius .
Locate the intersection points
Look at the points where the line and the circle intersect. These intersection coordinates are the solutions to the system.
Compare with the choices
Compare the coordinates of the intersection point(s) you see in Desmos with the answer choices and choose the option that matches exactly.
Step-by-step Explanation
Relate the two equations using
We know , so
On the other hand,
Since we are also given , we can connect these facts in the next step.
Find the product
Substitute into the identity:
But , so
Subtract 13 from both sides:
Now we know both the sum and the product .
Use sum and product to form a quadratic
If two numbers have sum 5 and product 6, they can be seen as the roots of a quadratic equation
So and are the roots of
Solve this quadratic to find the possible values of and .
Solve the quadratic and match to an ordered pair
Factor the quadratic:
So or , meaning and are and in some order. Both and satisfy the original equations, but among the answer choices only appears, so the correct ordered pair is .