1)
A = 1/(2x4) + 1/(4x6) + 1/(6x8) + 1/(8x10) + ....+ 1/(98x100)
2A= 2/(2x4) +2/(4x6) + 2/(6x8) + 2/(8x10) + ....+ 2/(98x100)
2A= 1/2 - 1/4 + 1/4- 1/6+ 1/6- 1/8+...+ 1/98 - 1/100
2A= 1/2 - 1/100
2A= 49/100
A= ($\frac{49}{100}$ ) : 2 = ($\frac{49}{100}$ ) . $\frac{1}{2}$
A= $\frac{49}{200}$
2> mình chưa biết làm......