Question 138·Hard·Equivalent Expressions
The identity
holds for all real numbers , where , and are integers.
The numbers , and are consecutive integers listed in increasing order.
What is the value of ?
For this kind of SAT question, recognize that a rational expression written as a polynomial plus a remainder over the same denominator comes from polynomial division. Multiply both sides by the denominator to get a polynomial identity, expand carefully, and then match coefficients of each power of to create equations linking the unknowns. Finally, use any extra conditions given (here, that are consecutive integers) to reduce the system and solve for the specific coefficient the question asks about. This coefficient-comparison method is usually faster and less error-prone than plugging in multiple -values.
Hints
Turn the complex fraction into polynomials
Because the equation is true for all , think about how to eliminate the denominator on both sides. What do you get if you multiply both sides by ?
Compare coefficients of like terms
After you clear the denominator and expand , line up the resulting polynomial with . How can you express , , and in terms of and ?
Use the fact that are consecutive
Translate "consecutive integers in increasing order" into equations that relate , , and . Then combine these with your earlier expressions for , , and in terms of and to get equations involving just and .
Solve the small system for and
You should end up with two equations that link and . Solve this system step by step—solve one equation for one variable, substitute into the other, and then find .
Desmos Guide
Check the identity with your values
After you have found specific integer values for , , , , and , type the left side as (x^3 + a*x^2 + b*x + c)/(x - 2) and the right side as x^2 + p*x + q + 5/(x - 2) in Desmos. If your work is correct, the two graphs should overlap completely (they represent the same function for all ); if they do not, recheck your algebra and the value of .
Step-by-step Explanation
Clear the denominator to get a polynomial identity
Because the equation holds for all real , you can multiply both sides by :
becomes
Now both sides are polynomials in , equal for all , so their coefficients must match.
Expand the right-hand side and match coefficients
First expand :
Then add the :
So
Matching coefficients of like powers of gives the system
- .
Use that are consecutive integers
The problem says are consecutive integers in increasing order, so
- .
Use from the previous step:
- .
But from coefficient matching you also have
- .
So you get two equations relating and :
- .
These two equations are enough to solve for and then .
Solve the system for and find
Solve :
Substitute this into :
Now plug back into :
So the value of is .