1)Tính các giới hạn sau
a)lim 4.3^n + 7^n+1/2.5^n +7^n
b) lim 2-x/(cănx+7)-3
x->2
1)Tính các giới hạn sau a)lim 4.3^n + 7^n+1/2.5^n +7^n b) lim 2-x/(cănx+7)-3 x->2
By Emery
By Emery
1)Tính các giới hạn sau
a)lim 4.3^n + 7^n+1/2.5^n +7^n
b) lim 2-x/(cănx+7)-3
x->2
Đáp án:
b. -6
Giải thích các bước giải:
\(\begin{array}{l}
a.\lim \dfrac{{{{4.3}^n} + {{7.7}^n}}}{{{{2.5}^n} + {7^n}}}\\
= \lim \dfrac{{4.{{\left( {\dfrac{3}{7}} \right)}^n} + 7}}{{2.{{\left( {\dfrac{5}{7}} \right)}^n} + 1}}\\
= \dfrac{7}{1} = 7\\
b.\mathop {\lim }\limits_{x \to 2} \dfrac{{2 – x}}{{\sqrt {x + 7} – 3}}\\
= \mathop {\lim }\limits_{x \to 2} \dfrac{{\left( {2 – x} \right)\left( {\sqrt {x + 7} + 3} \right)}}{{x + 7 – 9}}\\
= \mathop {\lim }\limits_{x \to 2} \dfrac{{\left( {2 – x} \right)\left( {\sqrt {x + 7} + 3} \right)}}{{x – 2}}\\
= \mathop {\lim }\limits_{x \to 2} \left( { – \sqrt {x + 7} – 3} \right)\\
= – 3 – 3 = – 6
\end{array}\)
`a) lim (4.3^{n} + 7^{n + 1})/(2.5^{n} + 7^{n})`
`= lim` $\dfrac{4.(\dfrac{3}{7})^{n} + 7}{2.(\dfrac{5}{7})^{n} + 1)}$
`= (0 + 7)/(0 + 1)`
`= 7`
`b) lim_{x -> 2} (2 – x)/(\sqrt{x + 7} – 3)`
`= lim_{x -> 2} ((2 – x)(\sqrt{x + 7} + 3))/(x + 7 – 9)`
`= lim_{x -> 2} (-sqrt{x + 7} – 3)`
`= -sqrt{2 + 7} – 3`
`= -6`