PSAT Math : How to find the solution for a system of equations

Study concepts, example questions & explanations for PSAT Math

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Example Questions

Example Question #2 : Creating Equations With Whole Numbers

\(\displaystyle 2x-y=2\)

\(\displaystyle x+y=4\)

What is the solution of \(\displaystyle x\) for the two systems of equations?

Possible Answers:

\(\displaystyle x=3\)

\(\displaystyle x=9\)

\(\displaystyle x=2\)

\(\displaystyle x=0\)

\(\displaystyle x=1\)

Correct answer:

\(\displaystyle x=2\)

Explanation:

We first add both systems of equations.

Left Hand Side:

\(\displaystyle (2x-y)+(x+y)=3x\)

Right Hand Side:

\(\displaystyle 2+4=6\)

Our resulting equation is:

\(\displaystyle 3x=6\)

 

We divide both sides by 3.

Left Hand Side:

\(\displaystyle \frac{3x}{3}=x\)

Right Hand Side:

\(\displaystyle \frac{6}{3}=2\)

Our resulting equation is:

\(\displaystyle x=2\)

Example Question #51 : How To Find The Solution For A System Of Equations

\(\displaystyle 4x+y=8\)

\(\displaystyle x+4y=17\)

What is the solution of \(\displaystyle y\) for the two systems?

Possible Answers:

\(\displaystyle y=6\)

\(\displaystyle y=3\)

\(\displaystyle y=1\)

\(\displaystyle y=2\)

\(\displaystyle y=4\)

Correct answer:

\(\displaystyle y=4\)

Explanation:

We first multiply the second equation by 4.

So our resulting equation is:

\(\displaystyle x\cdot4+4y\cdot4=17\cdot4\)

\(\displaystyle 4x+16y=68\)

Then we subtract the first equation from the second new equation.

Left Hand Side:

\(\displaystyle (4x+y)-(4x+16y)=-15y\)

Right Hand Side:

\(\displaystyle 6-68=-60\)

Resulting Equation:

\(\displaystyle -15y=-60\)

 

We divide both sides by -15

Left Hand Side:

\(\displaystyle \frac{-15y}{-15}=y\)

Right Hand Side:

\(\displaystyle \frac{-60}{-15}=4\)

Our result is:

\(\displaystyle y=4\)

 

Example Question #11 : How To Find The Solution For A System Of Equations

Find the solutions for the following set of equations:

 

\(\displaystyle 13x+2y=11\)

\(\displaystyle -5x-y=-1\)

Possible Answers:

\(\displaystyle x=2\)

\(\displaystyle y=-7.5\)

\(\displaystyle x=3\)

\(\displaystyle y=-14\)

\(\displaystyle x=1\)

\(\displaystyle y=-1\)

\(\displaystyle x=1\)

\(\displaystyle y=-4\)

\(\displaystyle x=\frac{9}{13}\)

\(\displaystyle y=1\)

Correct answer:

\(\displaystyle x=3\)

\(\displaystyle y=-14\)

Explanation:

If we multiply both sides of our bottom equation by \(\displaystyle 2\), we get \(\displaystyle -10x-2y=-2\). We can now add our two equations, and eliminate \(\displaystyle y\), leaving only one variable. When we add the equations, we get \(\displaystyle 3x=9\). Therefore, \(\displaystyle x=3\). Finally, we go back to either of our equations, and plug in \(\displaystyle x=3\) so we can solve for \(\displaystyle y\).

\(\displaystyle 13(3)+2y=11\)

\(\displaystyle 39+2y=11\)

\(\displaystyle 2y=-28\)

\(\displaystyle y=-14\) 

Example Question #11 : How To Find The Solution For A System Of Equations

Give the solution to the system of equations below.

\(\displaystyle 4x-3y=11\)

\(\displaystyle 2x+y=13\)

 

Possible Answers:

\(\displaystyle (6, 1)\)

\(\displaystyle (2, -1)\)

\(\displaystyle (4, -3)\)

No solution

\(\displaystyle (5, 3)\)

Correct answer:

\(\displaystyle (5, 3)\)

Explanation:

\(\displaystyle 4x-3y=11\)

\(\displaystyle 2x+y=13\)

Solve the second equation for \(\displaystyle y\), allowing us to solve using the substitution method.

\(\displaystyle 2x+y=13\)

\(\displaystyle y=13-2x\)

Substitute for  \(\displaystyle y\) in the first equation, and solve for \(\displaystyle x\).

\(\displaystyle 4x-3(13-2x)=11\)

\(\displaystyle 4x-39+6x=11\)

\(\displaystyle 10x-39=11\)

\(\displaystyle 10x=50\)

\(\displaystyle x=5\)

Now, substitute for \(\displaystyle x\) in either equation; we will choose the second. This allows us to solve for \(\displaystyle y\).

\(\displaystyle 2x+y=13\)

\(\displaystyle 2\cdot 5+y=13\)

\(\displaystyle 10+y=13\)

\(\displaystyle y=3\)

Now we can write the solution in the notation \(\displaystyle (x,y)\), or \(\displaystyle (5,3)\).

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