Đáp án:
bài 66
4 – (27 – 3) = x – (13 – 4)
4 – 27 + 3 = x – 13 + 4
4 – 27 + 3 + 13 – 4 = x
–27 + 16 = x
–11 = x.
Vậy x = –11.
bài 67:
a) (–37 ) + (–112)= –(37 + 112) = –149.
b) (–42) + 52= 52 – 42 = 10
c) 13 – 31 = –31 + 13 = – (31 – 13) = –18.
d) 14 – 24 – 12 = 14 – (24 + 12) = 14 – 36 = – (36 – 14) = –22.
e) (–25) + 30 – 15 = 30 – 25 – 15 = 30 – (25 + 15) = 30 – 40 = –10