Question 98·Medium·Systems of Two Linear Equations in Two Variables
In the solution to the system of equations above, what is the value of ?
(Express the answer as an integer)
For systems of two linear equations on the SAT, quickly choose between elimination and substitution by looking for easy matches in coefficients. If one variable’s coefficients can be made equal with a small multiplier, use elimination: align the equations, multiply if needed, then add or subtract to cancel one variable, solve for the remaining variable, and substitute back only if necessary. Since this question asks for just , stop as soon as you have , and double-check signs when subtracting the equations to avoid simple errors.
Hints
Look for an easier variable to eliminate
Compare the coefficients of and in both equations. Which variable can you make match more easily by multiplying one of the equations by a small number?
Match the x-coefficients
If you multiply the second equation by , what does the coefficient of become? Write that new equation under the first one.
Subtract to eliminate
With matching -coefficients, subtract one equation from the other. What expression do you get for the -terms, and what do you get on the right side?
Solve the resulting equation
You should now have an equation of the form . Solve this simple one-step equation for .
Desmos Guide
Enter the two equations
Type 6x - 2y = 14 on one line and 3x + 4y = 2 on the next line. Desmos will graph both lines on the coordinate plane.
Find the intersection point
Click on the point where the two lines intersect. Desmos will display the coordinates of this point; the second number is the value of that solves the system.
Step-by-step Explanation
Understand the goal
You are given a system of two linear equations and asked for the value of in the ordered pair that satisfies both equations:
You do not need to find if you can solve directly for .
Set up elimination to remove
Notice the coefficients of in the two equations are and . If we multiply the second equation by , the coefficient of will also be .
Multiply the entire second equation by :
Now you have a new system:
with matching -coefficients.
Eliminate by subtracting the equations
Subtract the first equation from the second to eliminate :
- Left side:
- Right side:
Compute each part:
So you get a single equation in :
Solve for y and answer the question
Solve by dividing both sides by :
So, in the solution to the system, the value of is .