We are given a quadratic function $f(x) = ax^2 + bx + c$, where $a, b, c$ are real numbers. The maximum value of $f(x)$ on the interval $-1 \le x \le 1$ is $M$, and the minimum value is $m$. We are given that $M = 1$ and $m = -1$. We need to find the coordinates of the vertex of the parabola $y = f(x)$ when $a = 1$, and determine the conditions on $b$ for $|-b/A| > 1$, which leads to $|b| > A$. Then we need to find the relationship between $|f(1) - f(-1)|$ and $|b|$ and $M - m$. Finally, we need to find the pairs $(b, c)$ that satisfy the given conditions.

AlgebraQuadratic FunctionsOptimizationVertex of a ParabolaInequalities
2025/6/13

1. Problem Description

We are given a quadratic function f(x)=ax2+bx+cf(x) = ax^2 + bx + c, where a,b,ca, b, c are real numbers. The maximum value of f(x)f(x) on the interval 1x1-1 \le x \le 1 is MM, and the minimum value is mm. We are given that M=1M = 1 and m=1m = -1. We need to find the coordinates of the vertex of the parabola y=f(x)y = f(x) when a=1a = 1, and determine the conditions on bb for b/A>1|-b/A| > 1, which leads to b>A|b| > A. Then we need to find the relationship between f(1)f(1)|f(1) - f(-1)| and b|b| and MmM - m. Finally, we need to find the pairs (b,c)(b, c) that satisfy the given conditions.

2. Solution Steps

(1) When a=1a = 1, f(x)=x2+bx+cf(x) = x^2 + bx + c. The vertex of the parabola y=f(x)y = f(x) is at x=b2x = -\frac{b}{2}. The yy-coordinate of the vertex is f(b2)=(b2)2+b(b2)+c=b24b22+c=cb24f(-\frac{b}{2}) = (-\frac{b}{2})^2 + b(-\frac{b}{2}) + c = \frac{b^2}{4} - \frac{b^2}{2} + c = c - \frac{b^2}{4}.
Thus, the vertex is (b2,cb24)(-\frac{b}{2}, c - \frac{b^2}{4}). Therefore, A = 2 and B =

4. If $|-\frac{b}{A}| > 1$, then $|-\frac{b}{2}| > 1$, which means $|\frac{b}{2}| > 1$, so $|b| > 2$. Thus A =

2. Now we calculate $|f(1) - f(-1)| = |(1^2 + b(1) + c) - ((-1)^2 + b(-1) + c)| = |1 + b + c - (1 - b + c)| = |2b| = 2|b|$.

Since M=1M = 1 and m=1m = -1, Mm=1(1)=2M - m = 1 - (-1) = 2.
Thus, f(1)f(1)=2b|f(1) - f(-1)| = 2|b|, and we are given f(1)f(1)=CbDMm(=2)|f(1) - f(-1)| = C |b| D M - m (= 2). Therefore C=2C=2. Also, we have 2b=2bD22|b|=2|b| D 2. Then, DD represents the equality. Thus DD corresponds to option 0 which is ==.
Since b>2|b| > 2, and M=1,m=1M = 1, m = -1 for 1x1-1 \le x \le 1, let us test the choices for (b, c).
Consider the case b>2|b| > 2.
If (b,c)=(1+23,1+23)(b, c) = (1 + 2\sqrt{3}, 1 + 2\sqrt{3}) (Option 0), b=1+23>2|b| = |1+2\sqrt{3}| > 2. Then f(x)=x2+(1+23)x+(1+23)f(x) = x^2 + (1+2\sqrt{3})x + (1+2\sqrt{3}).
If (b,c)=(123,123)(b, c) = (1 - 2\sqrt{3}, 1 - 2\sqrt{3}) (Option 1), b=12312(1.732)13.4642.464>2|b| = |1-2\sqrt{3}| \approx |1-2(1.732)| \approx |1-3.464| \approx 2.464 > 2. Then f(x)=x2+(123)x+(123)f(x) = x^2 + (1-2\sqrt{3})x + (1-2\sqrt{3}).
If (b,c)=(1+23,123)(b, c) = (-1 + 2\sqrt{3}, 1 - 2\sqrt{3}) (Option 2), b=1+231+3.4642.464>2|b| = |-1+2\sqrt{3}| \approx |-1+3.464| \approx 2.464 > 2. Then f(x)=x2+(1+23)x+(123)f(x) = x^2 + (-1+2\sqrt{3})x + (1-2\sqrt{3}).
If (b,c)=(123,1+23)(b, c) = (-1 - 2\sqrt{3}, 1 + 2\sqrt{3}) (Option 3), b=12313.4644.464>2|b| = |-1-2\sqrt{3}| \approx |-1-3.464| \approx 4.464 > 2. Then f(x)=x2+(123)x+(1+23)f(x) = x^2 + (-1-2\sqrt{3})x + (1+2\sqrt{3}).
The vertex is at x=b2x = -\frac{b}{2}. Since b>2|b| > 2, b2>1|-\frac{b}{2}| > 1. This means the vertex is outside the interval [1,1][-1, 1]. The maximum and minimum must occur at the endpoints x=1x=-1 and x=1x=1.
For option 3, f(1)=1+(123)+(1+23)=1f(1) = 1 + (-1-2\sqrt{3}) + (1+2\sqrt{3}) = 1, and f(1)=1(123)+(1+23)=1+1+23+1+23=3+43>1f(-1) = 1 - (-1-2\sqrt{3}) + (1+2\sqrt{3}) = 1 + 1 + 2\sqrt{3} + 1 + 2\sqrt{3} = 3 + 4\sqrt{3} > 1. Also, f(1)>1f(-1) > 1 and we have f(x)=x2(1+23)x+(1+23)f(x) = x^2 - (1+2\sqrt{3})x + (1+2\sqrt{3})
In this case the minimum must be at x=

1. $f(1) = 1$. Thus $f(-1) > 1$. So $x= -1$ is neither maximum or minimum.

For option 1, f(1)=1+(123)+(123)=34334(1.732)=36.928=3.928<1f(1) = 1 + (1-2\sqrt{3}) + (1-2\sqrt{3}) = 3-4\sqrt{3} \approx 3-4(1.732) = 3 - 6.928 = -3.928 < -1.
f(1)=1(123)+(123)=11+23+123=1f(-1) = 1 - (1-2\sqrt{3}) + (1-2\sqrt{3}) = 1 - 1 + 2\sqrt{3} + 1 - 2\sqrt{3} = 1
The vertex is at x=2312=30.51.7320.5=1.232>1x = \frac{2\sqrt{3} - 1}{2} = \sqrt{3} - 0.5 \approx 1.732 - 0.5 = 1.232 > 1.
Try (b,c)=(2+22,2+22)(b, c) = (2 + 2\sqrt{2}, 2 + 2\sqrt{2}). Then b=2+22>2|b| = 2 + 2\sqrt{2} > 2. Also f(x)=x2+(2+22)x+(2+22)f(x) = x^2 + (2 + 2\sqrt{2})x + (2+2\sqrt{2})
f(1)=1+2+22+2+22=5+42>1f(1) = 1 + 2 + 2\sqrt{2} + 2+2\sqrt{2} = 5 + 4\sqrt{2} > 1.
Consider M=1M=1 and m=1m=-1. Let E = 1, F =

2. $E < F$

3. Final Answer

A: 2
B: 4
C: 2
D: 0
E: 1
F: 2

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