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Question

Let $f(x) = x^{2025} - x^{2000}, x \in [0, 1]$ and the minimum value of the function $f(x)$ in the interval $[0, 1]$ be $(80)^{80} (n)^{-81}$. Then $n$ is equal to

The correct answer is
$-81$

Finding the Minimum Value of the Function

We need to find the minimum value of the function $f(x) = x^{2025} - x^{2000}$ on the interval $x \in [0, 1]$.

Step 1: Calculate the Derivative

First, find the derivative of $f(x)$ with respect to $x$.

$ f'(x) = \frac{d}{dx}(x^{2025} - x^{2000}) = 2025 x^{2024} - 2000 x^{1999} $

Step 2: Find Critical Points

Set the derivative equal to zero to find the critical points:

$ 2025 x^{2024} - 2000 x^{1999} = 0 $

Factor out the common term $x^{1999}$:

$ x^{1999} (2025 x^{25} - 2000) = 0 $

This yields two possibilities:

  • $x^{1999} = 0 \implies x = 0$
  • $2025 x^{25} - 2000 = 0 \implies 2025 x^{25} = 2000 \implies x^{25} = \frac{2000}{2025} = \frac{80}{81}

For the second case, $x = \left(\frac{80}{81}\right)^{\frac{1}{25}}$. This value lies within the interval $[0, 1]$. Let $x_c = \left(\frac{80}{81}\right)^{\frac{1}{25}}$.

Step 3: Evaluate the Function at Endpoints and Critical Points

Evaluate $f(x)$ at the endpoints ($x=0, x=1$) and the critical point ($x_c$).

  • At $x=0$: $f(0) = 0^{2025} - 0^{2000} = 0$.
  • At $x=1$: $f(1) = 1^{2025} - 1^{2000} = 1 - 1 = 0$.
  • At $x_c = \left(\frac{80}{81}\right)^{\frac{1}{25}}$:

    $ f(x_c) = \left(\left(\frac{80}{81}\right)^{\frac{1}{25}}\right)^{2025} - \left(\left(\frac{80}{81}\right)^{\frac{1}{25}}\right)^{2000} $

    $ f(x_c) = \left(\frac{80}{81}\right)^{81} - \left(\frac{80}{81}\right)^{80} $

    Factor out $\left(\frac{80}{81}\right)^{80}$:

    $ f(x_c) = \left(\frac{80}{81}\right)^{80} \left(\frac{80}{81} - 1\right) $

    $ f(x_c) = \left(\frac{80}{81}\right)^{80} \left(\frac{-1}{81}\right) $

    $ f(x_c) = \frac{-1}{81} \left(\frac{80}{81}\right)^{80} = (-81)^{-1} \cdot 80^{80} \cdot 81^{-80} = 80^{80} \cdot (-81)^{-81} $

Step 4: Determine the Minimum Value

Comparing the values $f(0)=0$, $f(1)=0$, and $f(x_c) = 80^{80} (-81)^{-81}$.

Since $x \in [0, 1]$, $x^{2025} \le x^{2000}$, which means $f(x) = x^{2025} - x^{2000} \le 0$. The minimum value must be negative.

Therefore, the minimum value is $f(x_c) = 80^{80} (-81)^{-81}$.

Step 5: Solve for n

The problem states that the minimum value is equal to $(80)^{80} (n)^{-81}$.

Equating the calculated minimum value with the given form:

$ 80^{80} (-81)^{-81} = (80)^{80} (n)^{-81} $

Divide both sides by $80^{80}$:

$ (-81)^{-81} = n^{-81} $

This implies:

$ n = -81 $

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