Question 30·Hard·Equivalent Expressions
Which of the following expressions is equivalent to
For rational expressions where a polynomial is divided by a linear factor like , think "polynomial division" right away: use synthetic division if the divisor is of the form to quickly get the quotient and remainder. Then write the original fraction as and match this structure to the answer choices, paying close attention to the sign and value of the remainder term; this is usually faster and less error-prone than expanding each option. If you are unsure, you can quickly check a candidate by multiplying it by the divisor to see whether you get back the original numerator.
Hints
Think about how to handle a polynomial over
You are dividing a cubic polynomial by a linear expression . How can you rewrite this as a polynomial plus a simpler fraction?
Use division rather than expanding the choices
Instead of expanding each answer choice, divide by (polynomial long division or synthetic division) to find the quotient and remainder.
After division, connect quotient and remainder to the answer form
Once you find the quotient and the remainder , write the original fraction as and see which choice has the same and the same sign on .
Desmos Guide
Enter the original expression
In Desmos, type
f(x) = (2x^3 - 5x^2 + 4x - 9) / (x - 2)
This is the function given in the problem.
Enter each answer choice as a separate function
Type the four options as
A(x) = 2x^2 - x + 2 + 5/(x - 2)B(x) = 2x^2 - x - 5 + 2/(x - 2)C(x) = 2x^2 + x + 2 - 5/(x - 2)D(x) = 2x^2 - x + 2 - 5/(x - 2)
so they all appear on the same graph.
Compare graphs or values to find the match
Zoom out if needed and see which of , , , or has a graph that lies exactly on top of the graph of (for all where they are defined). You can also use a table in Desmos (click the gear icon) and compare numerical values of and each candidate at several -values (such as ); the function that always matches is the equivalent expression.
Step-by-step Explanation
Understand what “equivalent expression” means here
We want to rewrite
in a different but mathematically equal form.
When you divide one polynomial by another linear polynomial like , you can always write
where is the quotient (a polynomial) and is the remainder (a constant). Our job is to find and , then pick the choice that matches .
Use synthetic division to divide by
Because the divisor is , use synthetic division with the number .
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Write the coefficients of the numerator in order:
- : coefficient
- : coefficient
- : coefficient
- constant : coefficient
So we use: .
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Set up synthetic division with on the left:
- Bring down the first coefficient: .
- Multiply (bottom) by (on the left): . Add to to get .
- Multiply by : . Add to to get .
- Multiply by : . Add to to get .
- The bottom row tells us:
- The quotient is the polynomial with coefficients , so .
- The remainder is .
Rewrite the fraction and match it to a choice
From polynomial division, we now know
So the expression is
Compare this with the answer choices: it matches choice D, .