Bài 1:
a) 42 + 37 + 58 + 63
= (42 + 58) + (37 + 63)
= 100 + 100
= 2 . 100
= 200
b) 199 + 16 + 201 + 84 + 37
= (199 + 201) + (16 + 84) + 37
= 400 + 100 + 37
= 537
c) 25 . 17 . 8 . 4 . 125
= (25 . 4) . (125 . 8) . 17
= 100 . 1000 . 17
= 1700000
d) 23 . 38 + 23 . 43 + 23 . 19
= 23 . (38 + 43 + 19)
= 23 . 100
= 2300
Bài 2:
a) x - 78 = 0
⇔ x = 0 + 78
⇔ x = 78
Vậy x = 78
b) (x - 79) . 108 = 0
⇔ x - 79 = 0 : 109
⇔ x - 79 = 0
⇔ x = 0 + 79
⇔ x = 79
Vậy x = 79
c) 108 . (79 - x) - 108 = 0
⇔ 108 . (79 - x) = 0 + 108
⇔ 108 . (79 - x) = 108
⇔ 79 - x = 108 : 108
⇔ 79 - x = 1
⇔ x = 79 - 1
⇔ x = 78
Vậy x = 78
d) (4 - x) . (6 - x) = 0
⇔ $\left \{ {{4 - x = 0} \atop {6 - x = 0}} \right.$
⇔ $\left \{ {{x = 4} \atop {x = 6}} \right.$
