Question

Tìm các giới hạn sau:

a) \(\lim \frac{{3n - 1}}{n}\)                                                   

b) \(\lim \frac{{\sqrt {{n^2} + 2} }}{n}\)      

c) \(\lim \frac{2}{{3n + 1}}\)                                                     

d) \(\lim \frac{{\left( {n + 1} \right)\left( {2n + 2} \right)}}{{{n^2}}}\)

Answer 1

a) \(\lim \frac{{3n - 1}}{n} = \lim \frac{{n\left( {3 - \frac{1}{n}} \right)}}{n} = \lim \left( {3 - \frac{1}{n}} \right) = 3 - 0 = 3\)

b) \(\lim \frac{{\sqrt {{n^2} + 2} }}{n} = \lim \frac{{\sqrt {{n^2}\left( {1 + \frac{2}{{{n^2}}}} \right)} }}{n} = \lim \frac{{n\sqrt {1 + \frac{2}{{{n^2}}}} }}{n} = \lim \sqrt {1 + \frac{2}{{{n^2}}}}  = 1 + 0 = 1\)

c) \(\lim \frac{2}{{3n + 1}} = \lim \frac{2}{{n\left( {3 + \frac{1}{n}} \right)}} = \lim \left( {\frac{2}{n}.\frac{1}{{3 + \frac{1}{n}}}} \right) = \lim \frac{2}{n}.\lim \frac{1}{{3 + \frac{1}{n}}} = 0.\frac{1}{{3 + 0}} = 0\)

d) \(\lim \frac{{\left( {n + 1} \right)\left( {2n + 2} \right)}}{{{n^2}}} = \lim \frac{{n\left( {1 + \frac{1}{n}} \right).2n\left( {1 + \frac{1}{n}} \right)}}{{{n^2}}} = \lim \frac{{2{n^2}{{\left( {1 + \frac{1}{n}} \right)}^2}}}{{{n^2}}}\)

                                                      \( = \lim 2{\left( {1 + \frac{1}{n}} \right)^2} = 2.{\left( {1 + 0} \right)^2} = 2\)