Question 79·Easy·Nonlinear Equations in One Variable; Systems in Two Variables
Consider the system of equations
Which ordered pair satisfies the system and lies in the first quadrant?
For systems with a line and a circle, the fastest algebraic method is substitution: solve the linear equation for one variable (often already done for you), plug that into the circle equation, and solve the resulting quadratic for . Then compute the matching -values and apply any extra conditions, such as quadrant restrictions ( and for the first quadrant). On SAT multiple-choice questions, you can also save time by quickly eliminating options that don’t match given conditions (like the quadrant) and then plugging the remaining choices into both equations to see which one satisfies the system.
Hints
Use the line equation to reduce the system
You are given . How can you use this to rewrite the circle equation with only in it?
Solve the resulting equation in one variable
After you substitute for , you will get a quadratic equation in . Factor or use the quadratic formula to find the possible -values.
Don’t forget the first-quadrant condition
For each -value you find, compute the corresponding using . Then ask: which point has both and positive?
Use answer choices and quadrants to check quickly
First quadrant means both coordinates are positive. Which answer choices have both and positive? Test just those in both equations.
Desmos Guide
Graph the line
In Desmos, type y = x + 1 to graph the straight line representing the first equation.
Graph the circle
On a new line in Desmos, type x^2 + y^2 = 25 to graph the circle centered at the origin with radius 5.
Find and interpret the intersection points
Use Desmos to click on the intersection points of the line and the circle; you should see two intersection coordinates. Identify which intersection has both and positive (first quadrant), then match that ordered pair to one of the answer choices.
Step-by-step Explanation
Understand what each equation represents
The equation is a straight line with slope and -intercept .
The equation is a circle centered at the origin with radius .
The solutions to the system are the points where the line and the circle intersect. The question also says the point must be in the first quadrant, meaning both and .
Use substitution to combine the equations
Since , substitute for in the circle equation:
Now expand :
So the equation becomes:
Combine like terms:
Move to the left side:
Divide everything by to simplify:
Solve the quadratic to find possible x-values
Factor the quadratic:
Look for two numbers that multiply to and add to . Those numbers are and , so:
Set each factor equal to :
Now find the corresponding -values using :
- If , then .
- If , then .
So the system has two intersection points: and .
Apply the first-quadrant condition and match the answer
The first quadrant consists of points where and .
- has both coordinates negative, so it is in the third quadrant, not the first.
- has both coordinates positive, so it is in the first quadrant.
Therefore, the ordered pair that satisfies the system and lies in the first quadrant is , which corresponds to choice D.