Precalculus : Write a Quadratic Equation When Given Its Solutions

Study concepts, example questions & explanations for Precalculus

varsity tutors app store varsity tutors android store

Example Questions

Example Question #1 : Write A Quadratic Equation When Given Its Solutions

Which of the following could be the equation for a function whose roots are at  and ?

Possible Answers:

Correct answer:

Explanation:

If the roots of the equation are at x= -4 and x=3, then we can work backwards to see what equation those roots were derived from. If we factored a quadratic equation and obtained the given solutions, it would mean the factored form looked something like:

Because this is the form that would yield the solutions x= -4 and x=3. If we work backwards and multiply the factors back together, we get the following quadratic equation:

Example Question #2 : Write A Quadratic Equation When Given Its Solutions

Given roots . Write a quadratic polynomial that has as roots.

Possible Answers:

Correct answer:

Explanation:

We can make a quadratic polynomial with  by mutiplying the linear polynomials they are roots of, and multiplying them out.

Start

  

Distribute the negative sign

 

FOIL the two polynomials. This means multiply the firsts, then the outers, followed by the inners and lastly, the last terms.

    

    

Simplify

    

 

Example Question #3 : Write A Quadratic Equation When Given Its Solutions

If we know the solutions of a quadratic equation, we can then build that quadratic equation.

Find the quadratic equation when we know that:

 and  are solutions.

Possible Answers:

Correct answer:

Explanation:

Since we know the solutions of the equation, we know that:

We simply carry out the multiplication on the left side of the equation to get the quadratic equation.

 

Example Question #4 : Write A Quadratic Equation When Given Its Solutions

Which of the following is a quadratic function passing through the points  and ?

Possible Answers:

Correct answer:

Explanation:

These two points tell us that the quadratic function has zeros at , and at .

These correspond to the linear expressions , and .

Expand their product and you arrive at the correct answer.

If the quadratic is opening up the coefficient infront of the squared term will be positive. Thus we get:

.

If the quadratic is opening down it would pass through the same two points but have the equation:

.

Since only  is seen in the answer choices, it is the correct answer.

Example Question #5 : Write A Quadratic Equation When Given Its Solutions

Which of the following roots will yield the equation .

Possible Answers:

 and 

 and 

Correct answer:

Explanation:

When we solve quadratic equations we get solutions called roots or places where that function crosses the x axis. Not all all will cross the x axis, since we have seen that functions can be shifted around, but many will. When they do this is a special and telling circumstance in mathematics. Since we know that roots of these types of equations are of the form x-k, when given a list of roots we can work backwards to find the equation they pertain to and we do this by multiplying the factors (the foil method).

If you were given an answer of the form  then just foil or multiply the two factors. If you were given only two x values of the roots then put them into the form   that would give you those two x values (when set equal to zero) and multiply to see if you get the original function. 

For our problem the correct answer is .

Use the foil method to get the original quadratic.

First multiply 2x by all terms in  :  

then multiply 2 by all terms in  .

We then combine    for the final answer. 

Example Question #6 : Write A Quadratic Equation When Given Its Solutions

Choose the quadratic equation that has these roots: 

 and 

Possible Answers:

Correct answer:

Explanation:

The roots or solutions of a quadratic equation are its factors set equal to zero and then solved for x. When roots are given and the quadratic equation is sought, write the roots with the correct sign to give you that root when it is set equal to zero and solved. For example, a quadratic equation has a root of -5 and +3. How could you get that same root if it was set equal to zero? With  and  because they solve to give -5 and +3. Thus, these factors, when multiplied together, will give you the correct quadratic equation.

Our roots were .

So our factors are  and .

Now FOIL these two factors:

 

First: 

Outer: 

Inner: 

Last: 

Simplify:

 

Example Question #7 : Write A Quadratic Equation When Given Its Solutions

Write the quadratic equation given its solutions.

 

Possible Answers:

None of these answers are correct.

Correct answer:

Explanation:

Therefore...

These two terms give you the solution. SO, work backwards.

FOIL (Distribute the first term to the second term).

Combine like terms...

Learning Tools by Varsity Tutors

Incompatible Browser

Please upgrade or download one of the following browsers to use Instant Tutoring: