Natural numbers only

( m 8 ) ( m 10 ) = 2 n (m-8)(m-10)=2^{n} m , n m,n are natural numbers. Find the number of ordered pairs ( m , n ) (m,n) such that the expression is satisfied.


The answer is 2.

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3 solutions

Jequil Hartz
Aug 30, 2015

Notice that ( m 8 ) (m-8) and ( m 10 ) (m-10) are factors of a power of 2 2 .

Since all factors of a power of 2 2 must be powers of 2 2 due to 2 2 being the only prime factor, ( m 8 ) (m-8) and ( m 10 ) (m-10) must be powers of 2 2 if m 8 m-8 and m 10 m-10 are assumed positive which occurs for values greater than ten.

Note that ( m 8 ) (m-8) and ( m 10 ) (m-10) also differ by 2 2 , The only 2 2 powers of 2 2 that differ by 2 2 are 2 2 and 4 4 , so the answers are whenever one of ( m 8 ) (m-8) and ( m 10 ) (m-10) is 2 2 and the other is 4 4 or whenever one of . ( 8 m ) (8-m) and ( 10 m ) (10-m) is 2 2 and the other is 4 4 . This is due to being able to rewrite the equation as ( 8 m ) ( 10 m ) = 2 n (8-m)(10-m) = {2}^{n} which would be the case where m 8 m-8 and m 10 m-10 are assumed negative which would occur for values less than 8.

This happens at m = 12 m = 12 or m = 6 m = 6 . Therefore, there are 2 2 ordered pairs satisfying this equation.

Moderator note:

Good observation. I like how you phrased the first line not as "are powers of 2", but as "are factors of a power of 2". This allows you to get the solution of m 8 = 2 m - 8 = - 2 .

Good observation. I like how you phrased the first line not as "are powers of 2", but as "are factors of a power of 2". This allows you to get the solution of m 8 = 2 m - 8 = - 2 .

Calvin Lin Staff - 5 years, 9 months ago

if m m is odd number then ( m 8 ) ( m 10 ) (m-8)(m-10) is odd number doesn't satisfy 2 n 2^{n} so m m is even number.

let m = 2 k m = 2k where k k is natural number than: ( m 8 ) ( m 10 ) = ( 2 k 8 ) ( 2 k 10 ) = 4 ( k 4 ) ( k 5 ) = 2 n (m-8)(m-10) = (2k-8)(2k-10) = 4(k-4)(k-5) = 2^{n} for satisfying this we need: ( k 4 ) (k-4) and ( k 5 ) (k-5) having the same sign and

( ( k 4 ) = 1 ( |(k-4)| = 1 and ( k 5 ) = 2 n 2 ) |(k-5)| = 2^{n-2})

or

( ( k 4 ) = 2 n 2 (|(k-4)| = 2^{n-2} and ( k 5 ) = 1 ) |(k-5)| = 1)

so k = 3 k=3 and k = 6 k=6 satisfy it m = 6 \Rightarrow m = 6 and m = 12 m=12

Hissam Karim
Sep 1, 2015

we have two factors (m-8) and (m-10).to satisfy the given condition,m should be such that both the factors give numbers which are (2,4,8 ,16.......).only 6 and 12 satisfy the condition.thus only two order pairs can be formed

Can you explain what you mean by "only 6 and 8 satisfy the condition, thus only tow order pairs can be formed"?

Calvin Lin Staff - 5 years, 9 months ago

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Sorry about the solution,i had wrongly typed.It's 12 and 6. Actually,12 and 6 ,when replace m give numbers which are generally represented by 2^n(here signs are not discussed because both the factors give same signs,e.g negative)....Moreover,n and m have one one relation,e.g for each value of m there is unique value of n)thus for two values of m there are two values of n.. and thus only two order pairs form

Hissam Karim - 5 years, 9 months ago

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How does m = 6 m = 6 "give numbers which are (2,4,8 ,16.......)"? In particular, we have m 8 = 2 , m 10 = 4 m-8 = -2, m - 10 = -4 are negative numbers, and not positive.

Also, why are these the only possibilities?

Calvin Lin Staff - 5 years, 9 months ago

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