Question 38·Medium·Systems of Two Linear Equations in Two Variables
Solve the following system of equations.
The solution to the system is . What is the value of ?
(Express the answer as an integer)
For systems of two linear equations, quickly scan the coefficients to choose the fastest method. If one variable has coefficients that are the same or opposites (like and here), use elimination by adding or subtracting the equations to remove that variable, solve for the remaining variable, then substitute back into either original equation to find the other variable. Always finish by checking which variable or expression the question actually asks for so you report the correct final quantity.
Hints
Look for an easy variable to eliminate
Compare the coefficients of and in both equations. Which variable has coefficients that are opposites, so that adding the equations will cancel it out?
Use elimination to find m first
Try adding the two equations together. This should give you an equation with only , which you can then solve.
Back-substitute to find n
Once you know , plug it into either original equation to get an equation in only, then solve that equation carefully.
Check the variable the question wants
After you solve the system, make sure you report the value of , not or both coordinates.
Desmos Guide
Rewrite each equation in y-form
Think of as and as . Rewrite the equations solving for :
- From : , so .
- From : , so .
In Desmos, you will use and instead of and .
Graph the first line
In the first expression line, type y = 13 - (3/2)x. This graphs the line that corresponds to .
Graph the second line
In the next expression line, type y = (3/4)x - 0.5. This graphs the line that corresponds to .
Find the intersection and read n
Zoom or pan until you see where the two lines intersect. Tap the intersection point; Desmos will display its coordinates . The -coordinate of this point is the value of that solves the system.
Step-by-step Explanation
Notice an easy elimination
Write the system:
The coefficients of are and . If you add the two equations, the terms will cancel (be eliminated).
Add the equations to solve for m
Add the left sides and the right sides:
This simplifies to:
Divide both sides by to get:
Substitute m back to get an equation in n
Use in one of the original equations, for example :
Simplify:
Now you have a single equation involving only . Solve this for .
Solve for n and answer the question
From the equation
subtract from both sides:
Divide both sides by :
The question asks for the value of , so the correct answer is .