Bài 1: Tính hợp lí
a) (427 – 2009) – (27 – 2009)
=-1582-(-1982)
=400
c) (18 + 29) + (158 – 18 – 29)
=47+(140-29)
=47+111
=158
b) 15 + 23 – 25 - 23
=(15-25)+(23-23)
=-10+0
=-10
d) (13 – 135 + 49) – (13 + 49)
=(-122+49)-62
=-73-62
=-135
Bài 2. Tìm x, biết
a) x + 13 = 32 – 76
x+13=-44
x=-44-13
x=-57
b) (-15) + x = (-14) – (-57)
(-15) + x =43
x=43-(-15)
x=58
c) 37 - (37- x) = 8 + (42 – 79)
37 - (37- x) = 8+(-37)
37 - (37- x) = -29
37-x=37-(-29)
37-x=66
x=37-66
x=-29
d) - 20 + (2x – 4) : 2 = 5
(2x – 4) : 2 = 5-2
(2x – 4) : 2 = 3
2x – 4 = 3.2
2x – 4  = 6
2x=6+4
2x=10
x=10:2
x=5