Properties of Log-Sums

Algebra Level 3

Let

A = n = 1 2014 n and B = n = 1 2014 n 3 . A= \sum_{n=1}^{2014}{n} ~~~\text{and} ~~~ B= \sum_{n=1}^{2014}{n^3}.

Find the value of log A B \log_{\sqrt A} {B}

Note : This question is taken from NTSE (Tamil Nadu) 2014.

2 3 4 1

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4 solutions

We have,

A = n = 1 2014 n = 1 + 2 + 3 + . . . + 2013 + 2014 = ( 2014 ( 2014 + 1 ) 2 ) = \displaystyle A = \sum_{n=1}^{2014} n = 1 + 2 + 3 + ... + 2013 + 2014 = \left(\frac{2014(2014+1)}{2}\right) =

something big = X \displaystyle= X

And,

B = n = 1 2014 n 3 = 1 3 + 2 3 + 3 3 + . . . . + 201 3 3 + 201 4 3 = ( 2014 ( 2014 + 1 ) 2 ) 2 = \displaystyle B = \sum_{n=1}^{2014} n^3 = 1^3 + 2^3 + 3^3 + .... + 2013^3 + 2014^3 = \left(\frac{2014(2014+1)}{2}\right)^{2} = something even bigger = X 2 \displaystyle = X^2

What we need is

l o g X X 2 = 2 1 / 2 l o g X X = 4 \displaystyle log_{ \sqrt{X}}{X^2} = \frac{2}{1/2}log_{X}{X} = \boxed{4}

And hence the answer!

cool man !

Hiếu Thái Ngọc - 6 years, 5 months ago

nice solution

Hafiz Rezoan - 6 years, 6 months ago

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@Hafiz Rezoan , Thank you!!

B.S.Bharath Sai Guhan - 6 years, 6 months ago

Thought it was log B A \log_{\sqrt{B}} A . *sigh *

Jake Lai - 6 years, 3 months ago

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I know that feel. Happens to the best of us :)

B.S.Bharath Sai Guhan - 5 years, 2 months ago

Good solution

Debmalya Mitra - 5 years, 11 months ago
Ryan Tamburrino
Nov 4, 2014

Cool fact: the square of a triangular number up to a number n is equal to the sum of the first n cubes!

Neelakash Biswas
Nov 6, 2014

A= n(n+1)/2 and B= (n(n+1)/2)^2 ,, i.e B=A^2 ,,, (logB)/(log(A)^0.5))=4

Vishu Jain
Nov 4, 2014

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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