Bài 3 :
a) $12.48-22.6.8$
$=12.48-22.48$
$=48.(12-22)$
$=48.(-10)$
$=-480$
b) $2486+(-486)-(-697)-(+97)+58$
$=(2485-486)+(679-97)+58$
$=2000+600+58$
$=2658$
Bài 4 :
a) $x-30=-20$
$⇔x=-20+30$
$⇔x=10$
b) $15+|x|=18$
$⇔|x|=3$
$⇔x=3,x=-3$
c) $-3<x<10, x∈Z$
$⇒x∈${$-2,-1,0,1,2,3,4,5,6,7,8,9$}