tính nhanh
h) 1999 . 7 + 1999 +199 . 2
i) \(\dfrac{1}{4} + \dfrac{1}{8} + \dfrac{1}{16} +\dfrac{1}{32} + \dfrac{1}{64} + \dfrac{1}{128}\)
k) \(\dfrac{1}{5} + \dfrac{1}{6} + \dfrac{1}{12} + \dfrac{1}{24} +\dfrac{1}{48} +\dfrac{1}{9}\)
l)\(\dfrac{1}{2.3} + \dfrac{1}{3.4} + \dfrac{1}{4.5} + \dfrac{1}{5.6} + \dfrac{1}{6.7} \)
n) 2,75 . 67 + 0,13 . 125 + 2,75 . 33 + 125 . 0,67
m) \(\dfrac{213.67+116}{213.68-97}\)
`h) 1999 . 7 + 1999 +1999 . 2`
`= 1999 xx (7 + 1 + 2)`
`= 1999 xx 10`
`= 19990`
Xin phép sửa `1` chút `:` `1/2+1/4+1/8+1/16+1/32+1/64+1/128`
Đặt `A=1/2+1/4+1/8+1/16+1/32+1/64+1/128`
`2A=2(1/2+1/4+1/8+1/16+1/32+1/64+1/128)`
`2A=1+1/2+1/4+1/8+1/16+1/32+1/64`
`2A-A=(1+1/2+1/4+1/8+1/16+1/32+1/64)-(1/2+1/4+1/8+1/16+1/32+1/64+1/128)`
`A=1-1/128`
`A=127/128`
`I)1/2.3+1/3.4+1/4.5+1/5.6+1/6.7`
`=1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7`
`=1/2-1/7`
`=5/14`
`n)2,75.67+0,13.125+2,75.33+125.0,67`
`=2,75.(67+33)+125.(0,13+0,67)`
`=2,75.100+125.0,8`
`=275+100`
`=375`
