a.
`347 + 478 + 123 + 12 + 1981`
`= (347 + 123) + (478 + 12) + 1981`
`= 470 + 490 + 1981`
`= 2941`
b.
`5*25*2*37*4`
`=(5*2)*(25*4)*37`
`=10*100*37`
`=37000`
c.
`125*2009*4:8*25`
`=(125*4)*2009:8*25`
`=500*2009:8*25`
`=(500:8)*2009*25`
`=62,5*2009*25`
`=1562,5*2009`
`=3139062,5`
d.
`38*63+37*38`
`=38*(63+37)`
`=38*100`
`=3800`
e.
`12*41+3*20:4+2*39*6`
`=12*41+15+468`
`=492+15+468`
`=975`
f.
`78*31+78*34+12*78+22*78`
`=78*(31+34+12+22)`
`=78*99`
`=78*(100-1)`
`=78*100-78`
`=7800-78`
`=7722`
g.
`85*(25-37)-35*(85-27)`
`=85*(-12)-35*58`
`=-1020-2030`
`=-3050`
h.
`11+12+13+...+50`
- Số số hạng là:
`(50 - 11) : 1 + 1 = 40` (số hạng)
- Tổng là:
`(50 + 11) * 40 : 2 = 1220`
