2/2×4 + 2/4×6 + ... + 2/16×18 + 2/18×20
= 1/2 - 1/4 + 1/4 - 1/6 + ... + 1/16 - 1/18 + 1/18 - 1/20
= 1/2 - 1/20
= 10/20 - 9/20
= 9/20
2/2x4+2/4x6+...+2/16x18+2/18x20
=1/2=1/4+1/4-1/6+...+1/16-1/18+1/18-1/20
=1/2-1/20
=10/20-9/20
=1/20
2/2x4 + 2/4x6 + .....+ 2/16x18 + 2/18x20
=1/2-1/4+1/4-1/6+...+1/18-1/20
=1/2-1/20
=9/20
\(\text{Ta có: }\) \(\frac{2}{2.4}+\frac{2}{4.6}+.....+\frac{2}{18.20}\)
\(=\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+....+\frac{1}{18}-\frac{1}{20}\)
\(=\frac{1}{2}-\frac{1}{20}\)
\(=\frac{9}{20}\)
