`A= 1/1.2 + 1/3.4 + 1/5.6 +...+1/49.50`
`A= 1/1 - 1/2 + 1/3 - 1/4 + 1/5 - 1/6 +...+ 1/49 - 1/50`
`A= (1 + 1/3 + 1/5 +....+ 1/49) -(1/2 +1/4 + 1/6+...+ 1/50)`
`A = ( 1 + 1/3 + 1/5+...+1/49) + (1/2 + 1/4 +1/6 +...+ 1/50) - (1/2 + 1/4 + 1/6 +...+1/50) - (1/2 + 1/4+ ...+1/50)`
`A= ( 1 + 1/2 + 1/3 + 1/4 +...+ 1/50) - 2( 1/2 + 1/4 +...+1/50)`
Mà
`B- 2C = 1 + 1/2 + 1/3 + 1/4 +...+1/50 - 2(1/2 + 1/4 + 1/6 +...+ 1/50)`
Vậy `A= B - 2C`
