The problem is about a quadratic function $y = ax^2 + bx + c$ and its graph. We need to determine the signs of $a$, $b$, and $c$ based on the graph, and then use the given equations to find the values of $a$, $c$ in terms of $b$. Finally, we need to find the range of possible values for $b$ when $a^2 - 8b - 8c$ is minimized.

AlgebraQuadratic FunctionsInequalitiesOptimizationVertex of a ParabolaGraph Analysis
2025/4/7

1. Problem Description

The problem is about a quadratic function y=ax2+bx+cy = ax^2 + bx + c and its graph. We need to determine the signs of aa, bb, and cc based on the graph, and then use the given equations to find the values of aa, cc in terms of bb. Finally, we need to find the range of possible values for bb when a28b8ca^2 - 8b - 8c is minimized.

2. Solution Steps

(1)
(i) Since the parabola opens downwards, a<0a < 0. The graph intersects the y-axis at a positive value, so c>0c > 0.
Since the axis of symmetry is on the positive side of y-axis, b/2a>0-b/2a > 0. Since a<0a < 0, this implies b>0b > 0.
So, AA is (9), BB is (7), and CC is (7).
(ii) The problem says a+b+c=0a+b+c = 0
Thus, DD is (0)
(iii) The problem says ab+c=0a-b+c=0
Thus, EE is (0)
(iv) The image does not give enough information to solve this equation.
(v) The image does not give enough information to solve this equation.
(2) From (i) and (ii), we have:
a+b+c=0a+b+c = 0
ab+c=0a-b+c = 0
Subtracting the second equation from the first, we get 2b=02b = 0, so b=0b = 0. However, we found that b>0b > 0 in part (1), and therefore, we cannot use the equation a+b+c=0a+b+c = 0 directly.
Since the problem mentions (i), we use a<0a < 0, b>0b > 0, and c>0c > 0. From (ii) and (iii), we have a+c=ba+c = -b and a+c=ba+c=b. Thus b=bb = -b, so 2b=02b=0, and b=0b=0.
This means our initial assessment was wrong.
We have a+b+c=0a+b+c = 0 and ab+c=0a-b+c=0. Then 2a+2c=02a+2c=0 or a+c=0a+c = 0.
Then a=ca = -c.
a28b8c=(c)28b8c=c28b8ca^2 - 8b - 8c = (-c)^2 - 8b - 8c = c^2 - 8b - 8c
Since a+b+c=0a+b+c = 0, a=bca=-b-c.
Since ab+c=0a-b+c=0, a=bca=b-c.
Thus bc=bc-b-c = b-c, so b=b-b = b, which implies 2b=02b = 0 or b=0b=0.
If b=0b = 0, we have a+c=0a+c = 0 or a=ca = -c. Then a28b8c=a28c=(c)28c=c28ca^2 - 8b - 8c = a^2 - 8c = (-c)^2 - 8c = c^2 - 8c.
The value is minimized when 2c8=02c - 8 = 0, so c=4c = 4.
Then a=4a = -4.
However, we need to express aa in terms of bb. If a=bca = b-c, then a=4a=-4, and b=0b=0, so c=4c = 4.
Since a=ca=-c, and a<0a<0 and c>0c>0, we consider the function f(a,b,c)=a28b8cf(a,b,c) = a^2 - 8b - 8c.
Since a+b+c=0a+b+c = 0, we have c=abc = -a - b.
Substituting for cc, we have a28b8(ab)=a28b+8a+8b=a2+8aa^2 - 8b - 8(-a-b) = a^2 - 8b + 8a + 8b = a^2 + 8a.
The minimum of a2+8aa^2 + 8a occurs when 2a+8=02a+8=0, so a=4a=-4.
Then HH is (6).
y=4x2+bx+c=4x2+bxb+Iy = -4x^2 + bx + c = -4x^2 + bx - b + I.
Since a+b+c=0a+b+c = 0, we have 4+b+c=0-4+b+c=0, so c=4bc = 4-b.
Therefore, y=4x2+bx+4by = -4x^2 + bx + 4-b.
Thus, II is (4).
Also b>0b>0. Since a+b+c=0a+b+c=0 and ab+c=0a-b+c=0, a+c=ba+c = -b, and a+c=ba+c=b, so 2b=02b = 0. This is not correct.
The vertex must be 0<x<10 < x < 1, so 0<b/(2a)<10 < -b/(2a) < 1. Since a<0a<0, we have 2a<b<02a < -b < 0, or 0<b<2a0 < b < -2a.
Since a=4a=-4, we have 0<b<80 < b < 8.
Since a=4a = -4, a<0a < 0. a+b+c=0a+b+c=0, 4+b+c=0-4+b+c=0, so c=4bc=4-b. Since c>0c>0, 4b>04-b>0, so b<4b<4.
Therefore 0<b<40<b<4.
JJ is (0), and KK is (4).

3. Final Answer

A: (9)
B: (7)
C: (7)
D: (0)
E: (0)
F: Don't Know
G: Don't Know
H: (6)
I: (4)
J: (0)
K: (4)

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