1 3 2 3 3 3 4 3 5 3 6 3 7 3 8 3 9 3 1 0 3 1 1 3 1 2 3 1 3 3 1 4 3 1 5 3 ⋮
Find the sum of numbers in the 4 5 th row.
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Nice Solution.
It would be better if you mention why you have chosen general term of the sequence ( 1 , 2 , 4 , 7 , 1 1 , ⋯ ) to be a quadratic function ( i.e. because the second order difference is constant ).
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Indeed @Karthik Venkata :)
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Better edit the solution to include this :).
The last numbers of each row are triangular number , i.e., 2 n ( n + 1 ) where n is the number of row. So I consider this more easier to interpret the first and last numbers of 4 5 t h row.
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First of all we need to determine the first number of the 4 5 t h row. The sequence of first numbers of each row is: 1 , 2 , 4 , 7 , 1 1 . . We can observe that the second order difference i.e. difference of differences is a constant value.
So lets consider a quadratic expression P ( n ) : a n 2 + b n + c
P ( 1 ) : a + b + c = 1 P ( 2 ) : 4 a + 2 b + c = 2 P ( 3 ) : 9 a + 3 b + c = 4
Solving the system of equations, we obtain a = 2 1 , b = 2 − 1 , c = 1 ⇒ P ( n ) : 2 1 n 2 − 2 1 n + 1 ⇒ P ( 4 5 ) = 9 9 1 .
The last number of the 4 5 t h row will be 9 9 1 + 4 4 = 1 0 3 5 .
Hence the 4 5 t h row is: 9 9 1 3 , 9 9 2 3 , 9 9 3 3 . . . 1 0 3 5 3 = r = 1 ∑ 1 0 3 5 r 3 − r = 1 ∑ 9 9 0 r 3 = ( 2 1 0 3 5 × 1 0 3 6 ) 2 − ( 2 9 9 1 × 9 9 2 ) 2 = 2 8 7 4 3 5 3 7 6 9 0 0 − 2 4 0 6 3 4 3 9 7 0 2 5 = 4 6 8 0 0 9 7 9 8 7 5