2023 AMC 12A Problems/Problem 20
Rows 1, 2, 3, 4, and 5 of a triangular array of integers are shown below.
Each row after the first row is formed by placing a 1 at each end of the row, and each interior entry is 1 greater than the sum of the two numbers diagonally above it in the previous row. What is the units digits of the sum of the 2023 numbers in the 2023rd row?
First, let be the sum of the th row. Now, with some observation and math instinct, we can guess that .
Now we try to prove it by induction,
(works for base case)
By definition from the question, the next row is always
Double the sum of last row (Imagine each number from last row branches off toward left and right to the next row), plus # of new row, minus 2 (minus leftmost and rightmost's 1)
Which gives us
Last, simply substitute , we get
Last digit of is ,
Let the sum of the numbers in row be . Let each number in row be where .
Then From this we can establish:
From this we have:
The problem requires us to find the last digit of . We can use modular arithmetic. ~luckuso
Solution 3 (Recursion)
Let the sum of the row be .
For each of the non-1 entries in the row, they are equal to the sum of the numbers diagonally above it in the row plus . Therefore all non-1 entries in the row appear twice in the sum of the non-1 entries in the row, the two s on each end of the row only appear once in the sum of the non-1 entries in the row. Additionally, additional s are placed at each end of the row. Hence,
, . By using the recursive formula,
is a geometric sequence by a ratio of
The unit digit of powers of is periodic by a cycle of digits: , , , . , the unit digit of is .
Therefore, the unit digit of is
Consider Pascal's triangle as the starting point. In the Pascal's triangle depicted below, the sum of the numbers in the th row is . For the 2023rd row in the Pascal's triangle, the sum of numbers is .
For the triangular array of integers in the problem, 1 is added to each interior entry, which propagates to two numbers diagonally below it in the next row, making the sum of numbers in the next row to increase by 2. And the addition of 1 continues to propagate to the next row, which makes the sum of numbers in the row below the next to increase by 4.
Examine the following triangular array of 1's, which indicates the 1's being added to each position of corresponding numbers in the Pascal's triangle.
For the 3rd row, 1 is added to the original number in the same position in the Pascal's triangle. And the addition of 1 in the 3rd row makes the sum of numbers in the 4th row to increase by 2, and makes the sum of numbers in the 5th row to increase by 4, and so forth. Therefore, the addition of a 1 in the 3rd row makes the sum of numbers in the 2023rd row to increase by . And similarly, the addition of each 1 in the 4th row makes the number of numbers in the 2023rd row to increase by . The 1's being added between the 3rd and 2022nd rows impact on the sum of numbers in the 2023rd row to increase by .
Therefore, the sum of numbers in the 2023rd row is the aggregation of the sum of numbers in the 2023rd row in the Pascal's triangle, the impact of addition of 1's between the 3rd row and the 2022nd row, and the addition of 1's on 2021 interior entries in the 2023rd row, which is .
Because and will always ends with 6 as the unit digit, the unit digit of is 4.
For , we can solve the sum of geometric sequence to be , which has 0 for the unit digit.
Therefore, for the sum of numbers in the 2023rd row, which is , its unit digit is .
Video Solution 1 by OmegaLearn
~Steven Chen (Professor Chen Education Palace, www.professorchenedu.com)
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