a, Lấy A-B
= 1+2+....+2^49+2^50 - 2^51
= 1+2+....+ 2^49+ 2^50 . ( 1-2)
= 1+2+.....+ 2^49 - 2^50
= 1+2+....+2^48 - 2^49
......
......
=   1+2+2^2-2^3
=   1+2-2^2
=   1-2 = -1 <0 ===>       A<B

A=1+2+3+4+5+...+99+100

A=(1+100).100:2=101.50=5050

B=1/2+1/6+1/12+1/20+1/30+...+1/9900

B=1/1.2+1/2.3+1/3.4+1/4.5+1/5.6+....+1/99.100

B=1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+...+1/99-1/100

B=1-1/100=99/100

Goi quang duong AB la x (km) dk (x>0)
Thoi gian luc di \(\frac{x}{60}\)
Thoi gian luc ve \(\frac{x}{50}\)
Biet rang thoi gian di it hon thoi gian ve la

\(\frac{x-1}{2}=\frac{y-2}{3}=\frac{z-3}{4}\Rightarrow\frac{2x-2}{4}=\frac{3y-6}{9}=\frac{z-3}{4}=\frac{2x+3y-z-2-6+3}{4+9-4}=\frac{27}{9}=3\)

\(\Rightarrow x=\frac{3\cdot4+2}{2}=7\)

\(\Rightarrow y=\frac{3\cdot9+6}{3}=11\)

\(\Rightarrow z=3\cdot4+3=15\)

a, P + O2 -) P2O5
c, SO3 + H2O-) H2SO4
d BaO + H2O -) Ba ( OH)2
e, CaCO3 -) Cao + CO2

cau b mik chuaa hieu lam