The problem asks to find the maximum value $M$ of the function $f(x) = |x^2 - 2x|$ in the interval $a \le x \le a+1$, where $a \ge 0$. We need to express $M$ in terms of $a$ and then find the minimum value of $M$ when $a$ varies over the non-negative real numbers.

AlgebraFunctionsAbsolute ValueMaximum ValueOptimizationIntervals
2025/4/14

1. Problem Description

The problem asks to find the maximum value MM of the function f(x)=x22xf(x) = |x^2 - 2x| in the interval axa+1a \le x \le a+1, where a0a \ge 0. We need to express MM in terms of aa and then find the minimum value of MM when aa varies over the non-negative real numbers.

2. Solution Steps

(1) First, let's express f(x)f(x) without the absolute value sign. We have x22x=x(x2)x^2 - 2x = x(x-2). Thus, x22x0x^2 - 2x \ge 0 when x0x \le 0 or x2x \ge 2, and x22x<0x^2 - 2x < 0 when 0<x<20 < x < 2. Therefore,
f(x)={x22xif x0 or x2x2+2xif 0<x<2f(x) = \begin{cases} x^2 - 2x & \text{if } x \le 0 \text{ or } x \ge 2 \\ -x^2 + 2x & \text{if } 0 < x < 2 \end{cases}
The graph of y=x22xy = x^2 - 2x is a parabola opening upwards with vertex at x=1x=1, and x22x=(x1)21x^2-2x = (x-1)^2 - 1. The vertex is at (1,1)(1,-1). The graph of y=x2+2xy=-x^2 + 2x is a parabola opening downwards with vertex at x=1x=1, and x2+2x=(x1)2+1-x^2+2x = -(x-1)^2 + 1. The vertex is at (1,1)(1,1).
So, we have M = 0, N =

2. Thus, $f(x) = x^2 - 2x$ when $x \le 0$ or $x \ge 2$ and $f(x) = -x^2 + 2x$ when $0 < x < 2$.

Now we consider the interval axa+1a \le x \le a+1.
Case 1: 0a10 \le a \le 1. Then the maximum value of f(x)f(x) occurs at either x=ax=a or x=a+1x=a+1 or x=1x=1.
When a=0a=0, 0x10 \le x \le 1, M=1M = 1, which occurs at x=1x=1.
When a=1a=1, 1x21 \le x \le 2, M=1M=1, which occurs at x=1x=1.
When 0a10 \le a \le 1, M=1M=1.
O = 1, P = 1
Case 2: 1<a21 < a \le 2. The maximum of f(x)f(x) for x[a,a+1]x \in [a, a+1] is attained either at aa or a+1a+1. In the case a+1<2a+1<2, M=max(2aa2,2(a+1)(a+1)2)=max(2aa2,1a2)M=\max(2a-a^2, 2(a+1)-(a+1)^2)= \max(2a-a^2, 1-a^2).
If a+1=2a+1=2, M=1a2M=1-a^2
f(a)=a2+2af(a) = -a^2 + 2a, f(a+1)=(a+1)22(a+1)f(a+1) = (a+1)^2-2(a+1). f(a+1)=(a+1)(a+12)=(a+1)(a1)=a21f(a+1) = (a+1)(a+1-2) = (a+1)(a-1) = a^2 - 1.
Since 1<a1 < a, consider f(a)=a2+2af(a) = -a^2+2a. Since a2a \le 2, consider f(a+1)=(a+1)22(a+1)=a21f(a+1) = (a+1)^2 - 2(a+1) = a^2 - 1. Thus, we want to compare a2+2a-a^2+2a and a21a^2-1.
If a2+2a>a21-a^2+2a > a^2-1, then 2a22a1<02a^2 - 2a - 1 < 0. a=2±4+84=2±124=1±32a = \frac{2 \pm \sqrt{4+8}}{4} = \frac{2 \pm \sqrt{12}}{4} = \frac{1 \pm \sqrt{3}}{2}. Since a>1a>1, we have 1<a<1+321 < a < \frac{1+\sqrt{3}}{2}.
a2+2a-a^2 + 2a if 1<a(1+3)/21 < a \le (1+\sqrt{3})/2, and a21a^2-1 if (1+3)/2<a2(1+\sqrt{3})/2 < a \le 2.
If a>2a > 2, then f(x)=x22xf(x) = x^2-2x, and M=max(a22a,(a+1)22(a+1))=max(a22a,a21)M = \max(a^2-2a, (a+1)^2-2(a+1)) = \max(a^2-2a, a^2-1).
f(a)=(a1)21=a22af(a) = (a-1)^2-1 = a^2 - 2a
f(a+1)=a21f(a+1) = a^2 - 1.
Case 3: a2a \ge 2. Then f(x)=x22xf(x) = x^2-2x in [a,a+1][a,a+1].
f(a)=a22af(a) = a^2-2a, f(a+1)=(a+1)22(a+1)=(a+1)(a1)=a21f(a+1) = (a+1)^2-2(a+1) = (a+1)(a-1) = a^2-1.
Then, we calculate a2+ta=a21-a^2 + ta = a^2 - 1 if and only if a>1+32a > \frac{1 + \sqrt{3}}{2}.
For x=1+32x= \frac{1+\sqrt{3}}{2}. Then, we have a22a=1a^2-2a = -1
The maximum occurs when x=(1+3)/2x=(1+\sqrt{3})/2, so Q=1Q=1, R=3R=3, S=2S=2.
M=a2+2aM = -a^2 + 2a when 1<a(1+3)/21 < a \le (1+\sqrt{3})/2.
T=2T=2
If a>(1+3)/2a > (1+\sqrt{3})/2, then f(a+1)>f(a)f(a+1) > f(a) and M=a21M=a^2-1.
U = 1
When 0a10\le a \le 1, M=1M=1.
When a2a \ge 2, M=a21M = a^2 - 1, which occurs at a+1a+1
M=a21M = a^2-1 is minimum when a=2a=2, then M=3M = 3.
If 1a21 \le a \le 2, M=a22aM = a^2 - 2a.
Consider case 1, 0a10 \le a \le 1. Since the function equals 11 in this case, the minimum value of MM is 11 between 00 and 11.
Let a=(1+3)/2a=(1+\sqrt{3})/2. Since 1<a1<a, we will study two regions separately.
When 1<a(1+3)/21 < a \le (1+\sqrt{3})/2, M=a2+2a=(a1)2+1M = -a^2 + 2a = -(a-1)^2 + 1. Since a>1a>1, this becomes a2+2a-a^2 + 2a.
When a=1a = 1, M=1M = 1.
When a=(1+3)/2a = (1+\sqrt{3})/2, a2+2a=(1+32)2+2(1+32)=1+23+34+1+3=4+234+1+3=132+1+3=32-a^2 + 2a = -(\frac{1+\sqrt{3}}{2})^2 + 2(\frac{1+\sqrt{3}}{2}) = -\frac{1+2\sqrt{3}+3}{4} + 1+\sqrt{3} = -\frac{4+2\sqrt{3}}{4} + 1+\sqrt{3} = -1 - \frac{\sqrt{3}}{2} + 1+\sqrt{3} = \frac{\sqrt{3}}{2}.
However, the first case is M=1M = 1. When a1a \rightarrow 1, the answer is approximately a1a - 1.
Also, we consider case when a>(1+3)/2a > (1+\sqrt{3})/2. Then M=a21M = a^2-1. When a=2a = 2, M=3M = 3.
When 1<a<(1+3)/21<a<(1+\sqrt{3})/2, M=a2+2aM=-a^2 + 2a is maximum when a=1a=1. At (1+3)/2(1+\sqrt{3})/2, the equation is 32\frac{\sqrt{3}}{2}.
Since we know when 0<a<10 < a < 1, M=1M=1. If we consider the function a2+2a-a^2+2a, the value is M=a2+2aM=-a^2+2a only if M1M \ge 1.
Then if 1<a<(1+3)/21 < a < (1+\sqrt{3})/2, then MM will start at 11 and decrease at 32\frac{\sqrt{3}}{2}.
However, for our case the smallest value of M is thus 3/2\sqrt{3} / 2 when M=1a2+2aM=-1a2 + 2a.
However, a<1a < 1 is M=1M = 1.
So V = 3\sqrt{3} and W = 2 is not it.
We need to recheck:
If a>1+32a > \frac{1+\sqrt{3}}{2} then a21a^2-1. It decreases fast but it doesn't affect at all.
The minimum value must happen when a=1+32a = \frac{1+\sqrt{3}}{2}. And 32<1\frac{\sqrt{3}}{2} <1. So the point when 32=M\frac{\sqrt{3}}{2}=M does NOT exist, but the problem is to when to change, so the answer might be V=3V=\sqrt{3}, and W=2W=2.

3. Final Answer

M = 0, N = 2
O = 1, P = 1
Q = 1, R = 3, S = 2
T = 2
U = 1
V = sqrt(3)
W = 2