Tính giới hạn của các dãy số sau :
a. \(\lim {{{n^4} - 40{n^3} + 15n - 7} \over {{n^4} + n + 100}}\)
b. \(\lim {{2{n^3} + 35{n^2} - 10n + 3} \over {5{n^5} - {n^3} + 2n}}\)
c. \(\lim {{\sqrt {6{n^4} + n + 1} } \over {2n + 1}}\)
d. \(\lim {{{{3.2}^n} - {{8.7}^n}} \over {{{4.3}^n} + {{5.7}^n}}}\)
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a. \(\lim {{{n^4} - 40{n^3} + 15n - 7} \over {{n^4} + n + 100}} = \lim {{1 - {{40} \over n} + {{15} \over {{n^3}}} - {7 \over {{n^4}}}} \over {1 + {1 \over {{n^3}}} + {{100} \over {{n^4}}}}} = 1\)
b. \(\lim {{2{n^3} + 35{n^2} - 10n + 3} \over {5{n^5} - {n^3} + 2n}} = \lim {{{2 \over {{n^2}}} + {{35} \over {{n^3}}} - {{10} \over {{n^4}}} + {3 \over {{n^5}}}} \over {5 - {1 \over {{n^2}}} + {2 \over {{n^4}}}}} = 0\)
c. \(\lim {{\sqrt {6{n^4} + n + 1} } \over {2n + 1}} = \lim {{{n^2}\sqrt {6 + {1 \over {{n^3}}} + {1 \over {{n^4}}}} } \over {n\left( {2 + {1 \over n}} \right)}} = \lim {{n.\sqrt {6 + {1 \over {{n^3}}} + {1 \over {{n^4}}}} } \over {2 + {1 \over n}}} \)
\(= + \infty \)
d. \(\lim {{{{3.2}^n} - {{8.7}^n}} \over {{{4.3}^n} + {{5.7}^n}}} = \lim {{3.{{\left( {{2 \over 7}} \right)}^n} - 8} \over {4{{\left( {{3 \over 7}} \right)}^n} + 5}} = - {8 \over 5}\)