Let's call this as the Mohanty's Triangle!

Logic Level 4

A triangle like the one shown above is constructed with the numbers from 1 to 2015 in the first row. Each number in the triangle, except those in the first row, is the sum of the two numbers above it. Let N N be the number that occupies the lowest vertex of the triangle.

If the prime factorization of N N is p 1 k 1 p 2 k 2 p 3 k 3 , p_1^{k_1} \cdot p_2^{k_2} \cdot p_3^{k_3}, find the value of p 1 + p 2 + p 3 + k 1 + k 2 + k 3 p_1 + p_2 + p_3 + k_1 + k_2 + k_3 .


The answer is 2033.

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5 solutions

Rohit Ner
Sep 13, 2015

In any n n sided Mohanty's triangle, any number in the triangle is 4 4 times of the number exactly above it. If there are 2015 2015 numbers in first row, the middle number is 1008 1008 . To obtain the number in the last row, we need to multiply the 1008 1008 by 4 4 1007 1007 times because there are 2014 2014 rows remaining out of which half the number have numbers exactly below 1008 1008 . Thus the number is 1008 × 4 1007 = 2 2018 . 3 2 . 7 1 1008\times {4}^{1007}\\\huge\color{#3D99F6}{=\boxed{{2}^{2018}.{3}^{2}.{7}^{1}}}

What would your method and answer be when there are 2016 numbers in the 1st row?

Kushagra Sahni - 5 years, 9 months ago

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For 2016 2016 numbers in the first row, 1008 1008 and 1009 1009 would be the middle terms. The number below them would be 2017 2017 . There are 2014 2014 rows remaining after the second row out of which 1007 1007 numbers would lie exactly below 2017 2017 . Hence, the last number would be 2017 × 4 1007 2017\times {4}^{1007} .

Rohit Ner - 5 years, 9 months ago

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Correct answer.

Kushagra Sahni - 5 years, 9 months ago

This is an Excellent question.

Kushagra Sahni - 5 years, 9 months ago
John Gilling
Sep 20, 2015

The bottom number in Mohanty's triangle starting with the numbers 1 through N on the top row comes out to be 2 N 2 ( N + 1 ) 2^{N-2}\cdot (N+1) . I originally saw this by looking at the leftmost entries of the kth row (which correspond to the bottom number of the triangle in the 1 through k case) and finding a pattern, then proving the general case via induction. The expression evaluates to 2 2013 2016 2^{2013} \cdot 2016 = 2 2013 2 5 3 2 7 1 = 2^{2013} \cdot 2^5 \cdot 3^2 \cdot 7^1 = 2 2018 3 2 7 1 . = 2^{2018} \cdot 3^2 \cdot 7^1. This yields the correct answer of 2033.

I also found the same pattern!

Deb Sen - 5 years, 8 months ago
Otávio Augusto
Feb 10, 2016

Let's consider the sequence of integers from 1 1 to 3 3 .

If you write the triangle in terms of the elements in the first row, you will get this:

1 2 3 1 + 2 2 + 3 1 + 2 2 + 3 { 1 \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 2 \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 3 \\ \ \ \ \ \ \ 1+2 \ \ \ \ \ \ \ \ \ 2+3 \\ \ \ \ \ \ \ \ \ 1+2 \cdot 2+3 }

I'm considering a small case but if you keep it going for sequences from 1 1 to 4 4 and from 1 1 to 5 5 for example, we can see that for a sequence from 1 1 to n n , the bottom number follows the formula:

i = 0 n 1 ( n 1 i ) ( i + 1 ) { \sum\limits_{i=0}^{n - 1} {n-1 \choose i} \cdot (i + 1) }

In this case, the bottom number is ( 2 0 ) ( 1 ) + ( 2 1 ) ( 2 ) + ( 2 2 ) ( 3 ) = 8 { {2 \choose 0} \cdot (1) + {2 \choose 1} \cdot (2) + {2 \choose 2} \cdot (3) = 8 } .

Tom Engelsman
Mar 8, 2017

If we simply focus on just the first and the last numbers of each row downward, we find that:

Left-hand side: 1, 3, 8, 20,... Right-hand side: 2015, 4029, 8056, 16108,...

with each side defined by the following recursions (for n 2 n \ge 2 ):

LHS: a n = 2 a n 1 + 2 n 2 ; a 1 = 1 a_n = 2a_{n-1} + 2^{n-2}; a_1 = 1

RHS: b n = 2 b n 1 2 n 2 ; b 1 = 2015 b_n = 2b_{n-1} - 2^{n-2}; b_1 = 2015

Let us now define a new sequence u n {u_n} such that u n = a n + b n ; u 1 = a 1 + b 1 = 2016. u_n = a_n + b_n; u_1 = a_1 + b_1 = 2016. Solving the recursion u n = 2 u n 1 u n = A 2 n u_n = 2u_{n-1} \Rightarrow u_n = A\cdot2^n , which the initial condition yields:

2016 = A 2 1 A = 1008 2016 = A\cdot2^{1} \Rightarrow A = 1008

or u n = 1008 2 n . u_n = 1008\cdot2^{n}. The bottom-most vertex value occurs at n = 2014 n = 2014 , which will give the prime factorization:

u 2014 = 1008 2 2014 = ( 2 4 3 2 7 1 ) 2 2014 = 2 2018 3 2 7 1 . u_{2014} = 1008\cdot2^{2014} = (2^{4}3^{2}7^{1})\cdot2^{2014} = 2^{2018}3^{2}7^{1}.

Hence, the required final sum is 2 + 2018 + 3 + 2 + 7 + 1 = 2 , 033 . 2 + 2018 + 3 + 2 + 7 + 1 = \boxed{2,033}.

Sonveer Yadav
Mar 8, 2016

Adding the first and last number of each row and writing them we get a g.p
(2016 , 4032 , 8064.........) with common ratio 2. Last term of it would give the lowest vertex number ( in the second last row we would have only two numbers, adding them would give the vertex no.) nth term of g.p = a × r n 1 a\times r^{n-1}
so, 2014th term of g.p = 2016 × 2 2013 2016\times 2^{2013} = ( 32 × 9 × 7 ) × 2 2013 (32 × 9 × 7)\times 2^{2013} = 2 2018 × 3 2 × 7 2^{2018}\times 3^{2}\times 7 now, 2+3+7+2018+2+1 = 2033

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