Question 225·Medium·Equivalent Expressions
Which of the following expressions is equivalent to
?
For expression-equivalence questions, first look for structure such as common factors or patterns instead of expanding everything. Here, recognize that both terms share , factor it out using the distributive property in reverse, then simplify the smaller expression inside the parentheses carefully, paying special attention to distributing minus signs and combining like terms. Only if no structure appears should you fall back on fully expanding and simplifying, which is slower and more error-prone on the SAT.
Hints
Look for a common factor
Both big products in the expression share the same binomial. Identify that shared binomial factor.
Use the distributive property in reverse
If you have something like , you can rewrite it as . Apply this idea to the given expression.
Handle the subtraction carefully
When you rewrite the expression inside the parentheses as , be sure to distribute the negative sign across both terms in .
Combine like terms
After distributing the minus sign, group the -terms together and the -terms together to simplify the expression inside the parentheses.
Desmos Guide
Enter the original expression
In a new Desmos line, type the original expression as a function of and , for example: A = (x+5y)(3x-2y) - (3x-2y)(4y-x).
Create sliders for x and y
Click on and in the expression to add sliders (or define x = 2 and y = 1 or any other values you like). These will let you quickly test numeric values.
Compare each choice numerically
For each answer choice, create a new line with its expression minus A. For example, for choice A type A1 = (3x-2y)(x+y) - A, for choice B type A2 = 6x^2 + x*y - 2y^2 - A, and similarly for the others. Then move the and sliders (or change their values) and see which line always evaluates to 0 for many different pairs. The choice whose difference stays 0 is equivalent to the original expression.
Step-by-step Explanation
Notice the common factor
The original expression is
Both terms contain the same binomial factor . This is like having , where is .
Factor using the distributive property
Use the distributive property in reverse: .
Here, let , , and .
So the expression becomes
Simplify the expression inside the brackets
Now simplify
First distribute the minus sign:
Combine like terms:
- Combine to get .
- Combine to get .
So the bracket simplifies to .
Write the fully simplified factored form
Substitute the simplified result back into the factored expression:
This matches answer choice D, so is the expression equivalent to the original one.