Find log 10 n m \log _{ 10 }{ { n }^{ m } }

Calculus Level 4

Let m m and n n be two distinct positive integers such that m < n < m 2 m<n<m^2 .

If the area of the region bounded between y = x m y = x^m , y = x n y = x^n , x = 0 x=0 and x = 1 x=1 is equal to 4 77 \dfrac4{77} , then which of the following is the value of log 10 n m \log_{10} n^m .

7 11 6 10

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

1 solution

Sanath Balaji
Jan 27, 2017

Nice question. I find that ( m , n ) = ( 18 , 1462 ) (m,n) = (18,1462) also works, which gives log 10 n m 56.97 \log_{10}n^{m} \approx 56.97 , which of course isn't one of the given options, so of the given options 6 6 is the unique answer.

Brian Charlesworth - 4 years, 4 months ago

Log in to reply

Thanks for the Information ,I did not knew such a possibility existed .

Sanath Balaji - 4 years, 4 months ago

Your solution is incomplete. You still haven't proved that 7, 10, 11 are not a possible value of log 10 n m \log_{10} n^m .

Pi Han Goh - 4 years, 4 months ago

Log in to reply

Let a = m + 1 a = m + 1 and b = n + 1 b = n + 1 . Then we have that

1 a 1 b = 4 77 77 b 77 a = 4 a b \dfrac{1}{a} - \dfrac{1}{b} = \dfrac{4}{77} \Longrightarrow 77b - 77a = 4ab \Longrightarrow

4 a b + 77 a 77 b = ( 2 a 77 2 ) ( 2 b + 77 2 ) + ( 77 2 ) 2 = 0 4ab + 77a - 77b = (2a - \frac{77}{2})(2b + \frac{77}{2}) + (\frac{77}{2})^{2} = 0 \Longrightarrow

( 77 4 a ) ( 4 b + 77 ) = 7 7 2 = 7 2 × 1 1 2 (77 - 4a)(4b + 77) = 77^{2} = 7^{2} \times 11^{2} .

Then since a , b > 0 a,b \gt 0 we need to distribute the factors of 7 7 2 77^{2} to 77 4 a 77 - 4a and 4 b + 77 4b + 77 in such a way that we get positive integer solutions for a , b a,b . For example,

77 4 a = 7 2 , 4 b + 77 = 1 1 2 ( a , b ) = ( 7 , 11 ) , ( m , n ) = ( 6 , 10 ) 77 - 4a = 7^{2}, 4b + 77 = 11^{2} \Longrightarrow (a,b) = (7,11), (m,n) = (6,10) ,

77 4 a = 1 , 4 b + 77 = 7 7 2 ( a , b ) = ( 19 , 1463 ) , ( m , n ) = ( 18 , 1462 ) 77 - 4a = 1, 4b + 77 = 77^{2} \Longrightarrow (a,b) = (19,1463), (m,n) = (18,1462) .

These turn out to be the only factor-splitting options that yield integer results, thus ruling out 7 , 10 , 11 7, 10, 11 as solutions to this problem.

Brian Charlesworth - 4 years, 4 months ago

I think this problem is overrated.

Md Zuhair - 4 years, 3 months ago

Log in to reply

You are also.

Sahil Silare - 3 years, 11 months ago

Log in to reply

Ya very true. I am quite overrated. I think i shall have been to calculus lvl 3 and others also. :)

Md Zuhair - 3 years, 11 months ago

Log in to reply

@Md Zuhair Lol level 4.

Sahil Silare - 3 years, 11 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...