Question 9-x-2-

2.2 Nonlinear Equations in One Variable - Quadratic solving (factoring, quadratic formula, completing square)
0:00

- 9 x 2 + 30 x + c = 0

In the given equation, c is a constant. The equation has exactly one solution. What is the value of c ?

A.

3

B.

0

C.

-25

D.

-53

- 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 -9x2+30x+c=0 has exactly one solution. A quadratic equation of the form a x 2 + b x + c = 0 has exactly one solution if and only if its discriminant, - 4 a c + b 2 , is equal to zero. It follows that for the given equation, a = -9 and b = 30 . Substituting -9 for a and 30 for b into b2-4ac  yields 302-4-9c, or 900+36c. Since the discriminant must equal zero, 900+36c=0. Subtracting 36 c from both sides of this equation yields 900 = - 36 c . Dividing each side of this equation by -36 yields -25 = c . Therefore, the value of c is -25

Choice A is incorrect. If the value of c is 3 , 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 c is 0 , 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 c is -53 , this would yield a discriminant that is less than zero. Therefore, the given equation would have no real solutions, rather than exactly one solution.