Question 14·Medium·Systems of Two Linear Equations in Two Variables
A system of equations is given by
What is the solution to the system?
For systems of two linear equations on the SAT, first look for an easy elimination: if the coefficients of one variable are opposites (like and ), add or subtract the equations to eliminate that variable and solve for the other. Then substitute this value into one original equation to find the remaining variable. Finally, quickly check the pair in both equations or plug the answer choices into the system if you are unsure—this ensures accuracy with minimal extra time.
Hints
Notice the structure of the equations
Look at the terms in the two equations. How are in the first equation and in the second equation related if you add the equations together?
Try eliminating a variable
Add the two equations term by term to eliminate one variable. What equation in terms of only do you get?
Use substitution to find the other variable
Once you know , plug it back into either original equation (pick the simpler one) and solve for .
Check your answer with both equations
After you get values for and , substitute them into both original equations to be sure they satisfy each one before choosing from the answer options.
Desmos Guide
Enter the first equation as a line
In Desmos, type the first equation solved for , for example y = (x + 7)/2. This will graph the line that represents .
Enter the second equation as a line
On a new line, type the second equation solved for , for example y = (13 - x)/3. This graphs the line that represents .
Find the intersection point
Look for the point where the two lines intersect. Tap or click that intersection point; Desmos will display its coordinates . Compare that ordered pair to the answer choices and select the matching one.
Step-by-step Explanation
Choose a method to solve the system
Notice that in the system
the terms have opposite signs ( and ). This makes the elimination (addition) method very convenient, because adding the equations will cancel .
Add the equations to eliminate and solve for
Add the left sides and the right sides of the two equations:
Now solve for by dividing both sides by to find the value of .
Substitute the value of back to find
Take the value you just found and substitute it into one of the original equations, for example .
- Replace with its value and simplify the left side.
- Solve the resulting simple equation for .
This gives you the corresponding value.
Write the solution as an ordered pair and match the choice
Combine the values you found into an ordered pair : and , so the solution is . This matches answer choice A.