Bài 5
$a, 5.(-7)+(-2).(-5)$
$=5.(-7)-5.(-2)$
$=5[-7-(-2)]$
$=5(-7+2)$
$=5.(-5)$
$=-25$
$b, 5.(-9)+(-2).3$
$=-45-6$
$=-51$
$c, 6.(-5)²+3.(-15)$
$=6.25-45$
$=150-45$
$=105$
$d, (-57).(67-34)-67.(34-57)$
$=(-57).33-67.(-23)$
$=-1881+1541$
$=-340$
$e, (-15):(-3)+3.(-8)$
$=5-24$
$=-19$
$f, (-21):3+3.(-7)$
$=-7+3.(-7)$
$=(-7)(1+3)$
$=(-7).4$
$=-28$
$g, S=2+(-3)+4+(-5)+6+(-7)+...+(-2019)+2020+(-2021)$
$=-1-1-1+...+(-1)-1$
$=-1010$
Bài 6
$a, 15-x=1-(-9)$
$⇔15-x=1+9$
$⇔15-x=10$
$⇔x=15-10$
$⇔x=5$
$b, x-10=5-(-7)$
$⇔x=5+7+10$
$⇔x=22$
$c, 30-x=2-(-8)$
$⇔30-x=2+8$
$⇔30-x=10$
$⇔x=30-10$
$⇔x=20$
$d, x-5=3-(-7)$
$⇔x=3+7+5$
$⇔x=15$
$e, |x|=10$
$⇔\left \{ {{x=10} \atop {-x=10}} \right.$
$⇔\left \{ {{x=10} \atop {x=-10}} \right.$
$f, |x-3|=10$
$⇔\left \{ {{x-3=10} \atop {3-x=10}} \right.$
$⇔\left \{ {{x=3} \atop {x=-7}} \right.$
$h, |x-7|=10$
$⇔\left \{ {{x-7=10} \atop {7-x=10}} \right.$
$⇔\left \{ {{x=7} \atop {x=-3}} \right.$
$k, |x|=5$
$⇔\left \{ {{x=5} \atop {-x=5}} \right.$
$⇔\left \{ {{x=5} \atop {x=-5}} \right.$
$i, |x+3|=15$
$⇔\left \{ {{x+3=15} \atop {-x-3=15}} \right.$
$⇔\left \{ {{x=12} \atop {-x=18}} \right.$
$⇔\left \{ {{x=12} \atop {x=-18}} \right.$
