Question 231·Medium·Equivalent Expressions
Which expression is equivalent to
for all real numbers with and ?
For equivalent-expression questions with rational expressions, first look for common factors in the numerator and denominator. Factor binomials and quadratics instead of expanding everything, then cancel any common factors while keeping domain restrictions in mind (values that make the denominator zero). Finally, rewrite the simplified form neatly and compare it directly to the answer choices rather than trying to manipulate the choices to fit the original expression.
Hints
Look at the structure of the numerator
Notice that both terms in the numerator involve . How can you factor out of the entire numerator?
Factor the numerator completely
After you factor out from the numerator, check whether the remaining factor can be simplified further by factoring out a constant.
Factor the denominator and compare
Factor into two binomials. Then see if any factor in the numerator also appears in the denominator so that the fraction can be simplified.
Remember the domain restrictions
When you cancel a factor, make sure that factor is never zero for the allowed values of (here, and ).
Desmos Guide
Enter the original expression
Type the original expression as a function, for example f(x) = ((3x+2)(x+5) - 5(x+5))/(x^2+6x+5) and press Enter. Be aware that the graph will have holes or undefined points at and .
Enter each answer choice as a separate function
Define each choice as its own function, for example g(x) = 3(x-1)/(x+1), h(x) = 3(x+1)/(x-1), etc. (one for each option), making sure to type them exactly as written.
Compare graphs or tables to find the match
Either (a) look at the graphs and see which choice’s graph lies exactly on top of the graph of (except at the excluded -values), or (b) use a table for each function and compare values of with each option at several -values (not equal to or ). The option whose values always match is the equivalent expression.
Step-by-step Explanation
Factor the numerator by taking out a common binomial
Look at the numerator:
Both terms contain , so factor out:
Now simplify inside the brackets:
so the numerator becomes .
Factor out the constant from the new numerator
Now factor out of :
So the entire numerator is
Factor the denominator
Factor the quadratic in the denominator:
We need two numbers that multiply to and add to . Those numbers are and , so
Now the whole fraction is
Cancel the common factor and match an answer choice
Since , the factor is never , so we can safely cancel from numerator and denominator:
This simplified expression matches answer choice A, so the correct answer is .