M=$\frac{1}{5}$+$\frac{1}{20}$+$\frac{1}{44}$+$\frac{1}{77}$+$\frac{1}{119}$+$\frac{1}{170}$
A=$\frac{1}{1.2}$+$\frac{1}{2.3}$+$\frac{1}{3.4}$+…+$\frac{1}{19.20}$
A=$\frac{1}{1.4}$+$\frac{1}{4.7}$+$\frac{1}{7.10}$…+$\frac{1}{16.19}$
Giải và cho mình cách làm nữa nhé =))
$\frac{1}{2}$ M = $\frac{1}{10}$+ $\frac{1}{40}$ +$\frac{1}{88}$ +$\frac{1}{154}$+ $\frac{1}{238}$+ $\frac{1}{340}$
= $\frac{1}{2.5}$+ $\frac{1}{5.8}$ +$\frac{1}{8.11}$ +$\frac{1}{11.14}$+ $\frac{1}{14.17}$+ $\frac{1}{17.20}$
=> $\frac{3}{2}$ M = 1/2 – 1/5 + 1/5 – 1/8 + 1/8 – 1/11 + 1/11 – 1/14 + 1/14 – 1/17 + 1/17 – 1/20
= 1/2 – 1/20 = 9/20
=> M = 3/10
A = 1 – 1/2 + 1/2 – 1/3 + 1/3 – 1/4 + … + 1/19 – 1/20
= 1 – 1/20 = 19/20
3A’ = 1 – 1/4 + 1/4 – 1/7 +…+ 1/16 – 1/19 = 1 – 1/19 = 18/19
=> A’ = 6/19.
1.M=2/2.(1/5+1/20+1/44+1/77+1/119+1/170)
=2.(1/10+1/40+1/88+1/154+1/238+1/340)
=2.(1/2.5+1/5.8+1/8.11+1/11.14+1/14.17+1/17.20)
=2.1/3(1/2-1/5+1/5-1/8+1/8-1/11+1/11-1/14+1/14-1/17+1/17-1/20)
=2/3.(1/2-1/20)
=2/3.9/20
=3/10
2.A=1/1-1/2+1/2-1/3+1/3-1/4+…+1/19-1/20
=1/1-1/20
=19/20
3.3A=1/1-1/4+1/4-1/7+1/7-1/10+…+1/16-1/19
3A=1/1-1/19
3A=18/19
A=18/19 chia 3
A=6/19
CHÚC BẠN HỌC TỐT=))