Question 2·Medium·Nonlinear Equations in One Variable; Systems in Two Variables
Consider the system of equations
What is the positive solution for that satisfies the system?
When you see a system where both equations are solved for and one is linear while the other is quadratic, immediately set the right-hand sides equal to create a single equation in . Rearrange into standard quadratic form, solve using factoring if it’s easy or the quadratic formula if not, and then use the question’s condition (such as asking for a positive solution, an integer solution, or a solution in a given interval) to select the appropriate root without wasting time checking both in detail.
Hints
Use the fact that both equations equal y
Since both equations describe in terms of , think about what you can do with and to find the -values where they are equal.
Form and recognize a quadratic equation
After you set the right sides equal, rearrange the equation so everything is on one side and the other side is zero. What kind of equation do you get, and what method can you use if it doesn’t factor nicely?
Remember to choose the correct solution
When you solve the quadratic, you will get two values for . Check which one is positive, because the question only wants the positive solution.
Desmos Guide
Graph the first equation
In Desmos, type y = 3x - 2 to graph the straight line.
Graph the second equation
On a new line in Desmos, type y = x^2 - 5 to graph the parabola.
Find the intersection points
Tap or click on each point where the line and the parabola intersect. Desmos will show the coordinates of these intersection points, including their -values.
Match the positive x-value to an answer choice
Look at the positive -coordinate of the intersection point. Then, in Desmos, type each answer choice expression (for example, (3 + sqrt(21))/2) on separate lines to see their decimal values, and choose the option whose value matches the positive intersection -coordinate.
Step-by-step Explanation
Set the two equations equal
Because both equations equal , set their right-hand sides equal to each other:
This equation will give the -values where the graphs intersect (the solutions to the system).
Rearrange into standard quadratic form
Move all terms to one side to get a standard quadratic equation:
So you need to solve . It does not factor nicely, so use the quadratic formula.
Apply the quadratic formula
For the quadratic , the solutions are given by
Here, , , and . Substitute these values:
This expression will simplify to two real solutions for .},{
Simplify and choose the positive solution
Now simplify the expression from the quadratic formula:
There are two solutions: and . The second one is negative (since ), and the problem asks for the positive solution. Therefore, the correct answer is , which corresponds to choice B.