Giải bài 30 trang 19 - SGK Toán lớp 7 Tập 1

a) $x : \left(-\dfrac{1}{2} \right)^3 = -\dfrac{1}{2}$
$\Leftrightarrow x = \left(-\dfrac{1}{2} \right).\left(-\dfrac{1}{2} \right)^3$
$\Leftrightarrow x = \left(-\dfrac{1}{2} \right)^4 = \dfrac{1}{16}$
b) $\left(\dfrac{3}{4} \right)^5 . x = \left(\dfrac{3}{4} \right)^7$
$\Leftrightarrow x = \left(\dfrac{3}{4} \right)^7 : \left(\dfrac{3}{4} \right)^5$
$\Leftrightarrow x = \left(\dfrac{3}{4} \right)^2 =\dfrac{9}{16}$

Lưu ý: 
$+ \,\,\,\, a^m : a^n = a^{m - n};\,\,a \ne 0, \, m \geq 0\\ + \,\,\,\, a^m.a^n = a^{m + n}$