7.
\(\lim\left(3.4^n-5^n\right)=\lim5^n\left(3.\left(\dfrac{4}{5}\right)^n-1\right)=+\infty.\left(-1\right)=-\infty\)
8.
\(\lim\dfrac{n^2+n-1}{3n+2}=\lim\dfrac{n^2\left(1+\dfrac{1}{n}-\dfrac{1}{n^2}\right)}{n\left(3+\dfrac{2}{n}\right)}=\lim\dfrac{n\left(1+\dfrac{1}{n}-\dfrac{1}{n^2}\right)}{3+\dfrac{2}{n}}=\dfrac{+\infty}{3}=+\infty\)
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