Tìm số tự nhiên n, biết a) <img alt="" src="http://img.loigiaihay.com/picture/article/2017/0316/bai-42-trang-23-sgk-toan-7-tap-1_1_1489637327.jpg" title="This is the rendered form of the eq...

a) $\dfrac{16}{2^n}=2\Rightarrow2^n=\dfrac{16}{2}=8=2^3$ $\Rightarrow n=3$ b) $\dfrac{\left(-3\right)^n}{81}=-27$ $\Leftrightarrow\dfrac{\left(-3\right)^n}{3^4}=\left(-3\right)^3$ $\Leftrightarrow\left(-3\right)^n=-\left(3^4\cdot3^3\right)=\left(-3\right)^7$ $\Rightarrow n=7$ c) $8^n:2^n=4$ $\Leftrightarrow2^{3n}:2^n=2^2$ $\Leftrightarrow3n-n=2$ $\Leftrightarrow2n=2\Rightarrow n=1$ Câu trả lời: a) $\dfrac{16}{2^n}=2\Rightarrow2^n=\dfrac{16}{2}=8=2^3$ $\Rightarrow n=3$ b) $\dfrac{\left(-3\right)^n}{81}=-27$ $\Leftrightarrow\dfrac{\left(-3\right)^n}{3^4}=\left(-3\right)^3$ $\Leftrightarrow\left(-3\right)^n=-\left(3^4\cdot3^3\right)=\left(-3\right)^7$ $\Rightarrow n=7$ c) $8^n:2^n=4$ $\Leftrightarrow2^{3n}:2^n=2^2$ $\Leftrightarrow3n-n=2$ $\Leftrightarrow2n=2\Rightarrow n=1$

a, $\dfrac{16}{2^n}=2\Rightarrow2^n.2=16\Rightarrow2^{n+1}=2^4\Rightarrow n+1=4\Rightarrow n=3$ . Vậy n=3. b, $\dfrac{\left(-3\right)^n}{81}=-27\Rightarrow\left(-3\right)^n=81.\left(-27\right)\Rightarrow\left(-3\right)^n=-2187$ $\Rightarrow\left(-3\right)^n=\left(-3\right)^7\Rightarrow n=7$ Vậy n=7 c, $8^n:2^n=4\Rightarrow\left(2^3\right)^n:2^n=2^2\Rightarrow2^{3n}:2^n=2^2\Rightarrow2^{2n}=2^2\Rightarrow2n=2\Rightarrow n=1$ Vậy n=1. Câu trả lời: a, $\dfrac{16}{2^n}=2\Rightarrow2^n.2=16\Rightarrow2^{n+1}=2^4\Rightarrow n+1=4\Rightarrow n=3$ . Vậy n=3. b, $\dfrac{\left(-3\right)^n}{81}=-27\Rightarrow\left(-3\right)^n=81.\left(-27\right)\Rightarrow\left(-3\right)^n=-2187$ $\Rightarrow\left(-3\right)^n=\left(-3\right)^7\Rightarrow n=7$ Vậy n=7 c, $8^n:2^n=4\Rightarrow\left(2^3\right)^n:2^n=2^2\Rightarrow2^{3n}:2^n=2^2\Rightarrow2^{2n}=2^2\Rightarrow2n=2\Rightarrow n=1$ Vậy n=1.