Question 48·Hard·Nonlinear Equations in One Variable; Systems in Two Variables
How many ordered pairs satisfy the system of equations above?
For nonlinear systems like a circle with a parabola, think in terms of intersection points. On the SAT (with a calculator), the fastest approach is usually to graph both equations and count how many intersection points appear. Without graphing, substitute one equation into the other to get a single equation in one variable, factor or approximate its real roots, and remember that each distinct real root gives one solution point on the system.
Hints
Think about the types of graphs
Identify what kind of graph each equation represents (for example, circle, line, parabola). How is the number of solutions related to how many times the graphs intersect?
Use substitution
Because the second equation already gives in terms of , try substituting into the first equation. This will give you a single equation in .
Focus on the resulting polynomial
After substitution, you will get a polynomial equation in . Factor out any simple roots you can find (like , , , , etc.), and then think about how many real roots remain.
Connect roots to solutions of the system
Each real solution for corresponds to one point on the parabola, and you already ensured it lies on the circle. How does the number of real -solutions relate to the number of ordered pairs that satisfy the system?
Desmos Guide
Enter the equations
In Desmos, type y = 2x - x^2 for the parabola and (x - 3)^2 + y^2 = 25 for the circle. Desmos will automatically plot both graphs.
Identify intersection points
Use the tap/click feature on the points where the parabola and circle cross. Desmos will highlight each intersection and display its coordinates.
Count the intersections
Count how many distinct intersection points appear between the two graphs. That count is the number of ordered pairs that satisfy the system.
Step-by-step Explanation
Recognize the graphs
Rewrite the equations in a more recognizable form:
- is a circle with center and radius .
- is a downward-opening parabola with vertex at .
The question is asking how many points these two graphs have in common (how many intersection points).
Use substitution to combine the equations
At any intersection point, the same must satisfy both equations.
Since , substitute this expression for into the circle equation:
Now the equation involves only , so solving it will give the -coordinates of intersection points.
Simplify to a polynomial in
Expand each term:
- .
- .
Substitute and set equal to :
Combine like terms and move to the left:
Now we need to find how many real solutions this quartic equation has.
Factor the polynomial and find its real roots
Look for an easy factor by testing small integer values. Try :
so is a root, and is a factor.
Divide by to get
We already have one real solution . Now consider the cubic :
- At : (negative).
- At : (positive).
Since the cubic is continuous and changes sign between and , it has one real root in that interval. (A cubic cannot have more than three real roots total, and a sign change on exactly one interval shows at least one and suggests isolating that one.)
Thus the quartic has exactly two real -values: and one value between and . Each gives exactly one from , so there are exactly 2 ordered pairs that satisfy the system. The correct choice is B) There are exactly 2 solutions.