ơia,(1-1/2)x(1-1/3)x(1-1/4)x(1-1/5)x….x(1-1/2003)x(1-1/2004)

 =1/2×2/3×3/4×4/5x…….x2002/2003×2004

=$\frac{1x2x3x4x…x2002x2003}{2x3x4x5x….x2003x2004}$ 

=$\frac{1}{2004}$ 

b,c=1/8+1/24+1/48+….+1/9800

 cx2=$\frac{2}{8}$+$\frac{2}{24}$ +…….+$\frac{1}{9800}$ 

 cx2=$\frac{2}{2×4}$+$\frac{2}{4×6}$+……+$\frac{2}{9800}$ 

 cx2=$\frac{1}{2}$-$\frac{1}{4}$+$\frac{1}{4}$-$\frac{1/6}{y}$+…….+$\frac{`1}{98}$-$\frac{1}{100}$

 cx2=$\frac{1}{2}$-$\frac{1}{100}$

 cx2=$\frac{49}{100}$ 

 c=$\frac{49}{100}$ :2

 C=$\frac{49}{200}$ 

c,1/2+1/4+1/8+1/16+….+1/512+1/1024

Bx2=1+($\frac{1}{2}$+$\frac{1}{4}$ +$\frac{1}{8}$ +……+$\frac{1}{512}$+$\frac{1}{1024}$)-$\frac{1}{1024}$ 

 Bx2=1+B-$\frac{1}{1024}$ 

 B=1-$\frac{1}{1024}$ 

 B=$\frac{1023}{1024}$ 

Chúc bạn học tốt<3