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
be the number that occupies the lowest vertex of the triangle.
If the prime factorization of N is p 1 k 1 ⋅ p 2 k 2 ⋅ p 3 k 3 , find the value of p 1 + p 2 + p 3 + k 1 + k 2 + k 3 .
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What would your method and answer be when there are 2016 numbers in the 1st row?
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For 2 0 1 6 numbers in the first row, 1 0 0 8 and 1 0 0 9 would be the middle terms. The number below them would be 2 0 1 7 . There are 2 0 1 4 rows remaining after the second row out of which 1 0 0 7 numbers would lie exactly below 2 0 1 7 . Hence, the last number would be 2 0 1 7 × 4 1 0 0 7 .
This is an Excellent question.
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 ) . 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 2 0 1 3 ⋅ 2 0 1 6 = 2 2 0 1 3 ⋅ 2 5 ⋅ 3 2 ⋅ 7 1 = 2 2 0 1 8 ⋅ 3 2 ⋅ 7 1 . This yields the correct answer of 2033.
I also found the same pattern!
Let's consider the sequence of integers from 1 to 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
I'm considering a small case but if you keep it going for sequences from 1 to 4 and from 1 to 5 for example, we can see that for a sequence from 1 to n , the bottom number follows the formula:
i = 0 ∑ n − 1 ( i n − 1 ) ⋅ ( i + 1 )
In this case, the bottom number is ( 0 2 ) ⋅ ( 1 ) + ( 1 2 ) ⋅ ( 2 ) + ( 2 2 ) ⋅ ( 3 ) = 8 .
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 ):
LHS: a n = 2 a n − 1 + 2 n − 2 ; a 1 = 1
RHS: b n = 2 b n − 1 − 2 n − 2 ; b 1 = 2 0 1 5
Let us now define a new sequence u n such that u n = a n + b n ; u 1 = a 1 + b 1 = 2 0 1 6 . Solving the recursion u n = 2 u n − 1 ⇒ u n = A ⋅ 2 n , which the initial condition yields:
2 0 1 6 = A ⋅ 2 1 ⇒ A = 1 0 0 8
or u n = 1 0 0 8 ⋅ 2 n . The bottom-most vertex value occurs at n = 2 0 1 4 , which will give the prime factorization:
u 2 0 1 4 = 1 0 0 8 ⋅ 2 2 0 1 4 = ( 2 4 3 2 7 1 ) ⋅ 2 2 0 1 4 = 2 2 0 1 8 3 2 7 1 .
Hence, the required final sum is 2 + 2 0 1 8 + 3 + 2 + 7 + 1 = 2 , 0 3 3 .
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
so, 2014th term of g.p =
2
0
1
6
×
2
2
0
1
3
=
(
3
2
×
9
×
7
)
×
2
2
0
1
3
=
2
2
0
1
8
×
3
2
×
7
now, 2+3+7+2018+2+1 = 2033
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In any n sided Mohanty's triangle, any number in the triangle is 4 times of the number exactly above it. If there are 2 0 1 5 numbers in first row, the middle number is 1 0 0 8 . To obtain the number in the last row, we need to multiply the 1 0 0 8 by 4 1 0 0 7 times because there are 2 0 1 4 rows remaining out of which half the number have numbers exactly below 1 0 0 8 . Thus the number is 1 0 0 8 × 4 1 0 0 7 = 2 2 0 1 8 . 3 2 . 7 1