Yahoo Answers is shutting down on May 4th, 2021 (Eastern Time) and beginning April 20th, 2021 (Eastern Time) the Yahoo Answers website will be in read-only mode. There will be no changes to other Yahoo properties or services, or your Yahoo account. You can find more information about the Yahoo Answers shutdown and how to download your data on this help page.

Show that for any integer n there exists an integer m such that:?

(sqrt(2)-1)^n =sqrt(m)-sqrt(m-1)

However, you are not allowed Newton's binomial expansion beyond (a+/-b)^2

Update:

Let's be clear. It is asked

a/ To exhibit an expression for m as a function of n, which is easy.

b/ To show that the corresponding expression indeed yields an integer. That is less easy. I found a solution using Newton's binomial expansion, but the problem was posed to young students who didn't have more than the three 'remarkable equalities' (as they say here), namely for (a+/-b)^2 and a^2-b^2.

Update 2:

Of course, they also know that (a^n)(b^n)=(ab)^n and that (a^n)^m = a^(nm)

4 Answers

Relevance
  • 1 decade ago
    Favorite Answer

    By taking inverses you would have

    (sqrt(2) + 1)^n = sqrt(m) + sqrt(m-1), so that

    2*sqrt(m) = (sqrt(2)-1)^n +(sqrt(2)+1)^n

    and the expression of m follows. I guess this is what you called a/ right?

    Can you use induction? If you can, then, simply by multiplying by

    (sqrt(2)-1)^2 + (sqrt(2)+1)^2,

    you see that the sequence m_n has the form

    m_(n+1) = 6*m_n - m_(n-1) - 2

    and so you get 2,9,50,289....... etc

    Incidentally, except for the (-2) this is the induction formula which yields the hypotenuses (w) in the Pythagorician triplets of the form (u,u+1,w)

    If they have an elementary way to see that (sqrt(2)-1)^n and (sqrt(2)+1)^n are of the form

    b sqrt(2) - a and b sqrt(2)+a when n is odd and

    - b sqrt(2) + a and b sqrt(2)+a when n is even

    then by multiplying one gets that

    2b^2 - a^2 is always + 1 or -1.

    So m = 2b^2 or a^2, depending on the parity.

  • bucci
    Lv 4
    4 years ago

    a million) if A is wonderful then A² is wonderful say, M = ok+a million and N = ok (ok : any beneficial integer) M²-N² = (ok+a million)² - ok² = 2k+a million (= any beneficial wonderful integer) so there are constantly 2 integers M and N at the same time with M²-N² : is any wonderful integer examples A = 15 = 2*7+a million ==> ok = 7 15 = 8² - 7² A² = 15² = 225 = 2*112 + a million ==> ok = 112 15² = 113² - 112² 2) if A is even then A² is even say, M = ok+a million and N = ok-a million (ok : any beneficial integer) M²-N² = (ok+a million)² - (ok-a million)² = 4k =2²ok A = 2B (B : integer) A² = 2²B² = 2²ok ==> ok = B² to fulfill the situation examples A = 18 = 2*9 ==> A² =18² = 2²*9² ==> ok = 80 one 18² = 80 two² - 80² A = 256 = 2*128 ==> A² =256² = 2²*128² ==> ok = 128²=16364 256² = 16365² - 16363² then for any integer A, we are able to discover a minimum of two integers M,N at the same time with A² = M² - N² ======================================...

  • 1 decade ago

    Okay, this is an re-edited answer. All powers of (√2 - 1) are of the form A + B, where A and B are either one of the two, one of which at least contains the radical √2:

    (1/2)( (1+√2)^x - (1-√2)^x )

    (1/2)( (1+√2)^x + (1-√2)^x )

    Proving that A² = B² ± 1 then proves this conjecture. Unfortunately, I am running out of time to explain fully.

  • Ken
    Lv 4
    1 decade ago

    Now I am not too sure about how to answer this, but here is my attempt:

    (√2 - 1)^n = √m - √(m-1)

    log (√2 - 1)^(√2 - 1)^n = log (√2 - 1)^(√m - √(m-1)

    n = log (√2 - 1)^(√m - √(m-1)

    It is then rewritten with natural log:

    n = ln(√m - √(m-1)) / ln(√2 - 1)

    Thus, I think it is proven that for any value "n", you can generate a real number consisting of the integer "m" (sense the bottom produces a constant). So was this what you were looking for?

Still have questions? Get your answers by asking now.