Question 8·Medium·Nonlinear Equations in One Variable; Systems in Two Variables
The ordered pair satisfies the system
If , what is the value of ?
(Express the answer as an integer)
For systems where you are given and , recognize that and are roots of the quadratic . Quickly write that quadratic, solve it by factoring (or using the quadratic formula if needed), then use any extra condition like to decide which root corresponds to which variable. This avoids solving nonlinear systems directly and is usually the fastest method on the SAT.
Hints
Think about sum and product
You are given and . Treat and as two numbers whose sum and product you know. How can you use both pieces of information together?
Relate to a quadratic equation
For two numbers with sum and product , they can be the roots of a quadratic equation of the form . What quadratic does that give you here?
Find the two numbers
Find two integers that add to and multiply to . Those will be the possible values for and .
Use the inequality
Once you have the two numbers, remember the condition to decide which one is .
Desmos Guide
Graph the related quadratic
In Desmos, enter the equation y = x^2 - 7x + 10. This quadratic has x-intercepts that correspond to the two numbers that satisfy the system.
Find the intercepts and apply
Click on the points where the graph crosses the x-axis; Desmos will display their x-coordinates. These two x-values are the possible values for and . Use the condition and choose the larger x-value as .
Step-by-step Explanation
Use the sum and product to form a quadratic
From the system, you know
For two numbers with sum and product , they are the roots of the quadratic
Here, and , so and are the roots of
Factor the quadratic (without choosing x yet)
Now factor the quadratic equation
Look for two integers that add to and multiply to . Call them and for now. Then the equation factors as
so the two possible values for (and therefore for and ) are and . You will decide which one is in the next step using .
Find the two numbers and use
The two integers that add to and multiply to are and , so the quadratic factors as
and the solutions are and .
This means .
Since the problem states , must be the larger of the two numbers.
Therefore, .