Question 179·Medium·Nonlinear Equations in One Variable; Systems in Two Variables
Consider the system of equations
The solution to the system is . What is the value of ?
(Express the answer as an integer)
For systems where one equation is exponential and the other is algebraic (like a quadratic), first solve the exponential equation if it is set equal to a simple constant. On the SAT, quickly rewrite the constant as a power of the same base (for example, write as ), equate the exponents to find , and then substitute that into the other equation to get . Work step by step with the arithmetic to avoid sign or multiplication errors, and remember you usually do not need to graph—algebraic manipulation is faster and more reliable under time pressure.
Hints
Start with the exponential equation
Look at first. Can you rewrite as a power of ?
Match the bases to solve for x
Once you write as , you will have . What can you say about the exponents when the bases are the same?
Use the value of x to find y
After you solve for from the exponential equation, substitute that into and simplify carefully.
Be careful with arithmetic
When you plug in , compute , then , and combine the numbers step by step to avoid sign errors.
Desmos Guide
Graph the equations to find x
In Desmos, enter y = 3^(x-2) and y = 9. Look for the point where these two graphs intersect and note the x-coordinate of that intersection; that is the value of that satisfies .
Use the x-value to compute y
In a new line, type y = x^2 - 6x + 13. Then either (a) click on the intersection point from step 1 and read off its x-coordinate, and substitute that value into x^2 - 6x + 13 in a separate expression, or (b) create a table for y = x^2 - 6x + 13 and enter that x-value in the table. The corresponding output is the -value that answers the question.
Step-by-step Explanation
Identify what the system is asking for
The system
represents all that satisfy both equations at the same time. To find , we first need to find the corresponding from the second equation and then plug that into the first equation.
Solve the exponential equation for x
Start with the second equation:
Rewrite as a power of :
So the equation becomes
Since the bases are the same and both sides are positive, set the exponents equal:
Solve for :
This is the -coordinate of the solution to the system.
Substitute x into the quadratic to find y
Now use the first equation, , and substitute :
Compute each part step by step:
- So
Simplify the expression for y
Combine the terms carefully:
So the solution to the system has -value
Therefore, the correct answer is .