Bài 1:
a) 5+(-7)+9+(-11)+13+(-15)
= 5-7+9-11+13-15
= (5-15)+(-7+13)+(9-11)
= -10+6+(-2)
= -4+(-2)=-6
b) 455+(-311)+3789+(-144)
= 455 -311+3789-144
= (455-144)+(-311+3789)
= 311+3478=3789
c)(2002-79+15)-(-79+15)
= 2002-79+15+79-15
= 2002+(-79+79)+[15+(-15)]
=2002+0+0=2002
d)-(-515-80+91)-(2003+80-91)
= 515+80-91-2003-80+91
= (515-2003)+(80-80)+[(-91)+91]
= -1488+0+0=-1488
Bài 2 :
a) 5x-35=-12-3
5x-35=-15
5x=-15+35
5x=20
x=20 : 5
x=4
b) 15-x=7-(-2)
15-x=7+2
15-x=9
x=15-9=6
c) x-(17- 8)=5+(10-3x)
x-9=5+10-3x
x-9=15-3x
x+3x=9+15
4x=24
x=24 : 4=6
d) 7+/x-5/=-8-(-26)
7+/x-5/=18
/x-5/=18-7
/x-5/=11
⇒x-5=11 ⇒x=11+5=16
x-5=-11 x=-11+5=-6
Bài 3 :
a)x+37+(-17)+25
=x+(37-17+25)
=x+45
b)(-66)-(-y-16)+150
=-66+y+16+150
=y+(-66+16+150)
=y+100
