1. **State the problem:**
Evaluate the sum
$$\frac{4}{2! + 3! + 4!} + \frac{5}{3! + 4! + 5!} + \frac{6}{4! + 5! + 6!} + \cdots + \frac{2025}{2023! + 2024! + 2025!}$$
2. **Rewrite the denominator:**
Given the hint, consider the general term for $n \geq 4$:
$$\frac{n}{(n-2)! + (n-1)! + n!}$$
3. **Factor the denominator:**
Note that
$$(n-1)! = (n-1)(n-2)!$$
$$n! = n (n-1)! = n (n-1)(n-2)!$$
So,
$$(n-2)! + (n-1)! + n! = (n-2)! + (n-1)(n-2)! + n (n-1)(n-2)!$$
$$= (n-2)! \left[1 + (n-1) + n(n-1)\right]$$
4. **Simplify the bracket:**
Calculate
$$1 + (n-1) + n(n-1) = 1 + n - 1 + n^2 - n = n^2$$
5. **Rewrite denominator:**
Therefore,
$$(n-2)! + (n-1)! + n! = (n-2)! \cdot n^2$$
6. **Rewrite the term:**
The term becomes
$$\frac{n}{(n-2)! \cdot n^2} = \frac{n}{n^2 (n-2)!} = \frac{1}{n (n-2)!}$$
7. **Express in terms of factorials:**
Recall that
$$n (n-2)! = n! / (n-1)$$
because
$$n! = n (n-1) (n-2)! \Rightarrow n (n-2)! = \frac{n!}{n-1}$$
So,
$$\frac{1}{n (n-2)!} = \frac{1}{\frac{n!}{n-1}} = \frac{n-1}{n!}$$
8. **Rewrite the sum:**
The sum is
$$\sum_{n=4}^{2025} \frac{n-1}{n!} = \sum_{n=4}^{2025} \frac{n-1}{n!}$$
9. **Split the numerator:**
$$\frac{n-1}{n!} = \frac{n}{n!} - \frac{1}{n!} = \frac{1}{(n-1)!} - \frac{1}{n!}$$
10. **Rewrite the sum using this:**
$$\sum_{n=4}^{2025} \left( \frac{1}{(n-1)!} - \frac{1}{n!} \right) = \sum_{n=4}^{2025} \frac{1}{(n-1)!} - \sum_{n=4}^{2025} \frac{1}{n!}$$
11. **Change index in the first sum:**
Let $k = n-1$, then when $n=4$, $k=3$, and when $n=2025$, $k=2024$:
$$\sum_{k=3}^{2024} \frac{1}{k!} - \sum_{n=4}^{2025} \frac{1}{n!}$$
12. **Combine sums:**
$$\left( \frac{1}{3!} + \frac{1}{4!} + \cdots + \frac{1}{2024!} \right) - \left( \frac{1}{4!} + \frac{1}{5!} + \cdots + \frac{1}{2025!} \right)$$
13. **Cancel common terms:**
All terms from $\frac{1}{4!}$ to $\frac{1}{2024!}$ cancel out, leaving
$$\frac{1}{3!} - \frac{1}{2025!}$$
14. **Calculate the final value:**
$$\frac{1}{3!} = \frac{1}{6}$$
Since $\frac{1}{2025!}$ is extremely small, the sum is approximately
$$\frac{1}{6} - \frac{1}{2025!} \approx \frac{1}{6}$$
**Final answer:**
$$\boxed{\frac{1}{6} - \frac{1}{2025!}}$$
This is the exact value of the sum.
Factorial Sum
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