Let
A = n = 1 ∑ 2 0 1 4 n and B = n = 1 ∑ 2 0 1 4 n 3 .
Find the value of lo g A B
Note : This question is taken from NTSE (Tamil Nadu) 2014.
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cool man !
nice solution
Thought it was lo g B A . *sigh *
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I know that feel. Happens to the best of us :)
Good solution
Cool fact: the square of a triangular number up to a number n is equal to the sum of the first n cubes!
A= n(n+1)/2 and B= (n(n+1)/2)^2 ,, i.e B=A^2 ,,, (logB)/(log(A)^0.5))=4
A=n(n+1)/2 B={n(n+1)/2}^2 Log√a b =log{ {n(n+1)/2}^0.5 {n(n+1)/2}^2 =4log n(n+1)/2 n(n+1)/2 =4
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We have,
A = n = 1 ∑ 2 0 1 4 n = 1 + 2 + 3 + . . . + 2 0 1 3 + 2 0 1 4 = ( 2 2 0 1 4 ( 2 0 1 4 + 1 ) ) =
something big = X
And,
B = n = 1 ∑ 2 0 1 4 n 3 = 1 3 + 2 3 + 3 3 + . . . . + 2 0 1 3 3 + 2 0 1 4 3 = ( 2 2 0 1 4 ( 2 0 1 4 + 1 ) ) 2 = something even bigger = X 2
What we need is
l o g X X 2 = 1 / 2 2 l o g X X = 4
And hence the answer!