Question 9-x-2-
In the given equation, is a constant. The equation has exactly one solution. What is the value of ?
- 9 x 2 + 30 x + c = 0 In the given equation, c
Hard-difficulty · SAT Math · Nonlinear Equations in One Variable — Quadratic solving (factoring, quadratic formula, completing square). Read the question above, select your answer, and check the full explanation below to understand exactly why the correct choice works.
Answer explanation
Choice C is correct. It's given that the equation has exactly one solution. A quadratic equation of the form has exactly one solution if and only if its discriminant, , is equal to zero. It follows that for the given equation, and . Substituting for and for into yields , or . Since the discriminant must equal zero, . Subtracting from both sides of this equation yields . Dividing each side of this equation by yields . Therefore, the value of is .
Choice A is incorrect. If the value of is , this would yield a discriminant that is greater than zero. Therefore, the given equation would have two solutions, rather than exactly one solution.
Choice B is incorrect. If the value of is , this would yield a discriminant that is greater than zero. Therefore, the given equation would have two solutions, rather than exactly one solution.
Choice D is incorrect. If the value of is , this would yield a discriminant that is less than zero. Therefore, the given equation would have no real solutions, rather than exactly one solution.
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