Question ID: 371cbf6b
Assessment Test Domain Skill Difficulty
SAT Math Advanced Math Equivalent expressions Hard
Question
The equation above is true for all x, where a and b are constants. What is the value of ab ?
Answer
A. 18
B. 20
C. 24
D. 40
Correct Answer: C
Rationale
Choice C is correct. If the equation is true for all x, then the expressions on both sides of the equation will be equivalent. Multiplying the
polynomials on the left-hand side of the equation gives . On the right-hand side of the equation,
the only -term is . Since the expressions on both sides of the equation are equivalent, it follows that ,
which can be rewritten as . Therefore, , which gives .
Choice A is incorrect. If , then the coefficient of on the left-hand side of the equation would be , which doesn’t
equal the coefficient of , , on the right-hand side. Choice B is incorrect. If , then the coefficient of on the left-hand side of
the equation would be , which doesn’t equal the coefficient of , , on the right-hand side. Choice D is incorrect. If
, then the coefficient of on the left-hand side of the equation would be , which doesn’t equal the coefficient of
, , on the right-hand side.