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Algebra Difficulty 6.3 National olympiad Find the answer

Let LL be the number formed by 20222022 digits equal to 11, that is, L=1111111L=1111\dots 111.
Compute the sum of the digits of the number 9L2+2L9L^2+2L.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Given a number L L consisting of 2022 digits, all equal to 1, we aim to compute the sum of the digits of the number 9L2+2L 9L^2 + 2L .

### Step 1: Express L L Numerically
The number L L can be expressed numerically as a sequence of ones, mathematically expressed as:
L=1111112022 ones=10202219 L = \underbrace{111\ldots111}_{2022\ \text{ones}} = \frac{10^{2022} - 1}{9}

### Step 2: Calculate 9L 9L
Multiply L L by 9, which simplifies the form:
9L=1020221 9L = 10^{2022} - 1
This is a number consisting of 2022 nines.

### Step 3: Calculate 9L2+2L 9L^2 + 2L
To find 9L2+2L 9L^2 + 2L , express (9L)2 (9L)^2 using the expression for 9L 9L :
9L2=(1020221)2=1040442×102022+1 9L^2 = (10^{2022} - 1)^2 = 10^{4044} - 2 \times 10^{2022} + 1
Now, add 2L 2L to this expression:
2L=2×10202219=2×(1020221)9 2L = 2 \times \frac{10^{2022} - 1}{9} = \frac{2 \times (10^{2022} - 1)}{9}
2L=2×10202229 2L = \frac{2 \times 10^{2022} - 2}{9}

Thus, we have that:
9L2+2L=1040442×102022+1+2×10202229 9L^2 + 2L = 10^{4044} - 2 \times 10^{2022} + 1 + \frac{2 \times 10^{2022} - 2}{9}
As seen above, both terms simplify upon combining:
9L2+2L=104044+1020221 9L^2 + 2L = 10^{4044} + 10^{2022} - 1

### Step 4: Sum the Digits of the Result
The number 104044+1020221 10^{4044} + 10^{2022} - 1 represents a number with a 1 at the first, a 1 at the 2023rd place, followed by zeros, and a trailing -1, producing all 9s thereafter until the last digit is 1.

For calculation, it produces a number of digits summing to:
9×2022+9×2021=4044 9 \times 2022 + 9 \times 2021 = 4044

Hence, the sum of the digits of 9L2+2L 9L^2 + 2L is:
4044 \boxed{4044}

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Source: Omni-MATH, licensed Apache-2.0. Statement and solution reproduced as published; topic and difficulty added by this site.