Question 217·Easy·Equivalent Expressions
Which of the following expressions is equivalent to the one above?
For products of polynomials on the SAT, immediately apply the distributive property: multiply each term in the first parentheses by each term in the second, write all resulting terms (with correct signs), then combine like terms carefully. Before you even multiply, use structure to eliminate impossible answers: here the product must be a cubic with no constant term and every term containing . As a quick check if you are unsure, you can also plug in a simple value like into both the original expression and the answer choices to see which one matches.
Hints
Recognize what is being asked
You are asked which option is equivalent to . That means you should expand (multiply out) the product and simplify it.
Use the distributive property
Think of it as distributing and then across . Multiply and by , then multiply them by .
Write all terms before combining
After distributing, you should have four terms. Write them all out carefully, including their signs, before you combine like terms (terms with the same power of ).
Check the structure of your result
Your final expression should be a cubic (highest power ) and every term should still contain , since the original expression has a factor of in .
Desmos Guide
Enter the original expression
In Desmos, type y1 = (x^2 + 5x)(2x - 3) to graph the original expression.
Enter each answer choice as a separate function
Add these lines in Desmos:
y2 = 2x^3 + 5x^2 - 15xy3 = 2x^3 - x^2 - 15xy4 = 2x^3 + 7x^2 + 15y5 = 2x^3 + 7x^2 - 15x
Compare the graphs to find the match
Turn the graphs on and off by tapping the circles next to each function. The correct equivalent expression is the one whose graph lies exactly on top of the graph of y1 = (x^2 + 5x)(2x - 3) for all visible -values; note which option that equation corresponds to.
Step-by-step Explanation
Set up the distribution
You need to expand .
Use the distributive property (often remembered as FOIL for binomials): every term in the first parentheses must be multiplied by every term in the second parentheses.
Multiply by the first term in the second parentheses
First, distribute across :
So from the you get: .
Multiply by the second term in the second parentheses
Next, distribute across :
So from the you get: .
Combine all terms and simplify
Now add together all four terms you found:
Combine like terms:
- The term:
- The terms:
- The term:
So the simplified expression is
This matches answer choice D.