Problems from Monotonous sequences

Development:

a) The sequence is not constant since $a_1=-8$ and $a_2=-{\displaystyle \frac{16}{3}}$.

To verify if the solution is increasing or decreasing it is sufficient to verify whether $a_n\le a_{n+1}$ or $a_n\ge a_{n+1}$, respectively.

Let's see if it is increasing. We want to verify if ${\displaystyle \frac{8n}{1-2n}}\le {\displaystyle \frac{8(n+1)}{1-(2n+1)}}$

By simplifying the factor $8$ and multiplying by the denominators, while observing that the denominators are always negative, we obtain $$n(1-2(n+1))\le (n+1)(1-2n)$$ By expanding the products we have $$-n-2n^2\le +1-2n^2-n$$

And by reducing the term $-n-2n^2$ we obtain $0\le 1$ which is always true independently of $n$. Therefore the sequence is increasing.

To see if the sequence is definitely increasing we must verify whether $a_n<a_{n+1}$.

We can verify it using the previous calculations about the solution being definitely increasing since the calculations that were carried out are true if we replace the inequality $\le$ by the strict inequality $<$.

b) The sequence is not constant since $b_1=1$ and $b_2={\displaystyle \frac{4}{5}}$.

Taking a look at the first two terms ot the sequence, it cannot be an increasing one. Let's see if the sequence is strictly decreasing. We verify if $${\displaystyle \frac{2n}{1+n^2}}>{\displaystyle \frac{2(n+1)}{1+(n+1{)}^2}}$$ Multiplying by the denominators; $$2n(1+(n+1{)}^2)>(1+n^2)2(n+1)$$ Expanding we obtain; $$4n+4n^2+2n^3>2+2n+2n^2+2n^3$$ Simplifying we obtain $$n^2+n-1>0$$ Computing two roots of the previous polynomial we see that they are both smaller than $1$. Therefore, for $n$ integer it is satisfied that $n^2+n-1>0$ and the sequence is definitely decreasing.

c) The sequence is not constant since $c_1=-{\displaystyle \frac{3}{2}}$ and $c_2=-2$.

By taking a look at the first two terms it can happen that the sequence is definitely decreasing. We verify if $${\displaystyle \frac{n^2+2}{-n-1}}>{\displaystyle \frac{(n+1{)}^2+2}{-(n+1)-1}}$$ By multiplying by the denominators we obtain $$(-n-2)(n^2+2)>(n^2+2n+3)(-n-1)$$ By multiplying by $-1$, and therefore inverting the inequality and expanding, we obtain; $$n^3+2n^2+2n+4<n^3+3n^2+5n+3$$ By subtracting we obtain the inequality $$n^2+3n-1>0$$

By calculating the roots we see that the two of them are smaller than 1 and, therefore, the inequality is true for every integer $n$.

Therefore, the sequence is strictly decreasing.

Solution:

a) The sequence is strictly increasing.

b) The sequence is strictly decreasing.

c) The sequence is strictly decreasing.

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