Question 158·Medium·Equivalent Expressions
Constants and satisfy the identity, valid for all real :
What is ?
When you see an identity involving rational expressions that must hold for all values of (except where the denominator is zero), first clear the denominator by multiplying both sides by it. Then expand to write both sides as standard polynomials and match coefficients of like powers of to solve for unknown constants. Finally, be sure to answer exactly what is asked—often a sum or difference of those constants—rather than stopping at the individual values.
Hints
Remove the fraction
Try multiplying both sides of the equation by so that you no longer have a denominator. What does the equation become?
Distribute carefully
After you clear the denominator, you will get on one side. Expand this product step by step and then simplify.
Use coefficient matching
Once both sides are written as standard polynomials in , compare the coefficients of and the constant terms on both sides to solve for and .
Answer the question asked
After you find and , remember the problem asks for , not or separately.
Desmos Guide
Set up both sides in Desmos
In one line, enter f(x) = (x^3 - 3x^2 + kx + m)/(x - 2) and let Desmos create sliders for k and m. In another line, enter g(x) = x^2 - x + 1 + 5/(x - 2).
Match the graphs to determine and
Adjust the sliders for k and m until the graphs of f(x) and g(x) overlap everywhere they are both defined (except at where there is a hole). The slider values at that point are the correct and .
Check and compute the sum
After you have the correct k and m from the sliders, verify that the graphs still coincide for several different -values, then add those two values (by hand or using Desmos) to find .
Step-by-step Explanation
Clear the denominator
The equation
is stated to hold for all real , so we can safely multiply both sides by :
Now both sides are polynomials in .
Expand the right-hand side
Expand :
Now add the :
So the equation becomes
Match coefficients to find and
Because these polynomials are equal for all real , their coefficients for each power of must match.
Compare the coefficients:
- Coefficient of : (already matches).
- Coefficient of : (already matches).
- Coefficient of : .
- Constant term: .
So and .
Compute the requested sum
The question asks for .
So the correct answer is .