Đáp án:
Giải thích các bước giải:
Bài 1:
`a)(279-1987)+(-18+1987-279`
`=279-2987-18+1987-279`
`=-18`
`b)-(3251+415)-(2000+585-251)`
`=-666-2334`
`=-3000`
`c)-25-26-27-28-29+15+16+17+18+19`
`=-10+(-10)+(-10)+(-10)(-10)`
`=-50`
`d)71-(-30)-18+(-30)+118`
`=71+30-18-30+118`
`=71-18+118`
`=71-100=-29`
Bài 9:
`a)2x+3=x+5`
`⇔2x-x=-3+5`
`⇔x=2`
`c)-x+8+2x=-x+16`
`⇔-x+2x+x=-8+16`
`⇔2x=8`
`⇔x=4`
`b)(x+5)+(x-9)=x+2`
`⇔x+5+x-9=x+2`
`⇔x+x-x=-5+9+2`
`⇔x=6`
`d)2x-(30+x)=x-(27-8)`
`⇔25-30-x=x-19`
`⇔-x-x=-25+10-19`
`=-2x=-34`
`⇔x=17`
`e)(x-12)-15=(20-7)-(18+x)`
`⇔x-12-15=13-18-x`
`⇔x+x=12+15+13-18`
`⇔2x=22`
`⇔x=11`
`f)(15-x)+(x-12)=7-(-8+x)`
`⇔15-x+x-12=7+8-x`
`⇔-x+x+x=-15+12+7+9`
`⇔x=13`
