Question the-pa

2.1 Nonlinear Functions - Quadratic graphs and vertex form
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  • The parabola opens upward.
  • The vertex is at point (negative 1 comma negative 8).
  • The parabola passes through the following points:
    • (negative 2 comma negative 6)
    • (negative 1 comma negative 8) 
    • (0 comma negative 6)

The graph of y = 2 x 2 + b x + c is shown, where b and c are constants. What is the value of b c ?

Enter your answer:

The parabola opens upward. The vertex is at point (negative 1 comma negative 8). The parabola

Hard-difficulty · SAT Math · Nonlinear Functions — Quadratic graphs and vertex form. Read the question above, select your answer, and check the full explanation below to understand exactly why the correct choice works.

Answer explanation

The correct answer is -24 . Since the graph passes through the point 0,-6, it follows that when the value of x is 0 , the value of y is -6 . Substituting 0 for x and -6 for y in the given equation yields -6=202+b0+c, or -6=c. Therefore, the value of c is -6 . Substituting -6 for c in the given equation yields y=2x2+bx-6. Since the graph passes through the point -1,-8, it follows that when the value of x is -1 , the value of y is -8 . Substituting -1 for x and -8 for y in the equation y=2x2+bx-6 yields -8=2-12+b-1-6, or -8=2-b-6, which is equivalent to -8=-4-b. Adding 4 to each side of this equation yields -4=-b. Dividing each side of this equation by -1 yields 4=b. Since the value of b is 4 and the value of c is -6 , it follows that the value of bc is 4-6, or -24 .

Alternate approach: The given equation represents a parabola in the xy-plane with a vertex at -1,-8. Therefore, the given equation, y=2x2+bx+c, which is written in standard form, can be written in vertex form, y=ax-h2+k, where h,k is the vertex of the parabola and a is the value of the coefficient on the x2 term when the equation is written in standard form. It follows that a=2. Substituting 2 for a , -1 for h , and -8 for k in this equation yields y=2x--12+-8, or y=2x+12-8. Squaring the binomial on the right-hand side of this equation yields y=2x2+2x+1-8. Multiplying each term inside the parentheses on the right-hand side of this equation by 2 yields y=2x2+4x+2-8, which is equivalent to y=2x2+4x-6. From the given equation y=2x2+bx+c, it follows that the value of b is 4 and the value of c is -6 . Therefore, the value of bc is 4-6, or -24 .