Don't make mistakes! #5

Calculus Level 3

Sam's teacher gave him a question. The question was:

\quad Find the value of lim n ( n 2 6 n + 3 n 2 4 n + 7 ) \displaystyle \lim_{n\rightarrow\infty}\left(\sqrt{n^2-6n+3}-\sqrt{n^2-4n+7}\right) .

Now he is going to try and find the answer.

Sam's solution:

\quad 1: lim n 2 n 2 10 n + 9 n 2 = lim n 2 ( n 2 6 n + 3 ) ( n 2 4 n + 7 ) n 2 \displaystyle \lim_{n\rightarrow\infty}\frac{2n^2-10n+9}{n^2} = \lim_{n\rightarrow\infty} \frac{2\sqrt{(n^2-6n+3)(n^2-4n+7)}}{n^2}

\quad 2: lim n ( 2 n 2 10 n + 9 ) = lim n 2 ( n 2 6 n + 3 ) ( n 2 4 n + 7 ) \displaystyle \lim_{n\rightarrow\infty}\left(2n^2-10n+9\right) = \lim_{n\rightarrow\infty} 2\sqrt{(n^2-6n+3)(n^2-4n+7)}

\quad 4: lim n ( 2 n 2 10 n + 10 2 ( n 2 6 n + 3 ) ( n 2 4 n + 7 ) ) = 1 \displaystyle \lim_{n\rightarrow\infty}\left(2n^2-10n+10-2\sqrt{(n^2-6n+3)(n^2-4n+7)}\right)=1

\quad 8: lim n ( n 2 6 n + 3 n 2 4 n + 7 ) 2 = 1 \displaystyle \lim_{n\rightarrow\infty}\left(\sqrt{n^2-6n+3}-\sqrt{n^2-4n+7}\right)^2=1

\quad 16: If n > 6 , n 2 6 n + 3 < n 2 4 n + 7 \displaystyle n>6,\ \sqrt{n^2-6n+3}<\sqrt{n^2-4n+7}

\quad 32: Therefore lim n ( n 2 6 n + 3 n 2 4 n + 7 ) = 1 \displaystyle \lim_{n\rightarrow\infty}\left(\sqrt{n^2-6n+3}-\sqrt{n^2-4n+7}\right)=-1

Find the sum of all the numbers of the individual equations that are not true. If you think all of the given equations are true, submit 0 as your answer.


This problem is a part of <Don't make mistakes!> series .


The answer is 2.

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1 solution

Kalpok Guha
Jul 4, 2017

Second step is not correct.As we cannot cancel lim n 1 n 2 \lim_{n\rightarrow\infty}\frac{1}{n^2} in that way.

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