\(=\dfrac{1}{2}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{6}+...+\dfrac{1}{50}-\dfrac{1}{52}=\dfrac{1}{2}-\dfrac{1}{52}=\dfrac{25}{52}\)

Đặt \(A=2.4+4.6+...+50.52\)

\(\Rightarrow6A=2.4.6+4.6.6+...+50.52.6\)

\(=2.4.6+4.6.\left(8-2\right)+....+50.52.\left(54-48\right)\)

\(=2.4.5+4.6.8-2.4.6+...+50.52.54-48.50.52\)

\(=50.52.54\)

\(\Rightarrow A=\frac{50.52.54}{6}=23400\)

\(\text{Đặt A = 2.4 + 4.6 + ... + 50.52}\)

\(\text{⇒6A = 2.4.6 + 4.6.6 + ... + 50.52.6}\)

\(\text{6A= 2.4.6 + 4.6. 8 − 2 + .... + 50.52. 54 − 48 }\)

\(\text{6A= 2.4.5 + 4.6.8 − 2.4.6 + ... + 50.52.54 − 48.50.52}\)

\(\text{6A=50.52.54 }\)

\(\Rightarrow A=\)\(\frac{\text{50.52.54 }}{6}\)\(=23400\)

chịu

\(\dfrac{2}{2.4}+\dfrac{2}{4.6}+................+\dfrac{2}{50.52}\)

\(=\dfrac{1}{2}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{6}+..............+\dfrac{1}{50}-\dfrac{1}{52}\)

\(=\dfrac{1}{2}-\dfrac{1}{52}=\dfrac{25}{52}\)

\(\dfrac{2}{2.4}+\dfrac{2}{4.6}+.....+\dfrac{2}{50.52}\)

\(=\dfrac{1}{2}-\dfrac{1}{4}+\dfrac{1}{4}-\dfrac{1}{6}+.....+\dfrac{1}{50}-\dfrac{1}{52}\)

\(=\dfrac{1}{2}-\dfrac{1}{52}=\dfrac{25}{52}\)

tự làm đi thớt bài này quá dễ rồi

Xét tử: 2.4+4.8+8.12+12.16+16.20 = 2.1.2.2 + 2.2.2.4 + 2.2.4.6 + 2.2.6.8 + 2.2.8.10

                                                   = 2.2.(1.2+2.4+4.6+6.8+8.10)

=> 2.4+4.8+8.12+12.16+16.20/1.2+2.4+4.6+6.8+8.10 = 2.2.(1.2+2.4+4.6+6.8+8.10) / 1.2+2.4+4.6+6.8+8.10

                                                                               = 2.2 = 4

camon nhan (:

A=1/2.4+1/4.6+........+1/100.102

A=1/2-1/4+1/4-1/6+.......+1/100-1/102

A=1/2-1/102

A=51/102-1/102

A=50/102

A=25/51

\(A=\frac{4}{2.4}+\frac{4}{4.6}+...+\frac{4}{2014.2016}=\frac{1}{2}+\frac{1}{6}+...+\frac{1}{1015056}=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{1007}-\frac{1}{1008}\)

\(=1-\frac{1}{1008}=\frac{1007}{1008}\)

4/2-4/4+4/4-4/6+....+4/2014-4/2015

=4/2-4/2015

=2-4/2015

= 4030-4/2015

=4026/2015

A=4/2.4+4/4.6+4/6.8+...+4/2008.2010

=2.(2/2.4+2/4.6+2/6.8+...+2/2008.2010)

=2.(1/2-1/4+1/4-1/6+1/6-1/8+...+1/2008-1/2010)

=2.(1/2-1/2010)

=2.502/1005

=1004/1005

Vậy A=1004/1005

100% giải đúng đầu tiên:

       Ta có: \(A=\frac{4}{2.4}+\frac{4}{4.6}+\frac{4}{6.8}+...+\frac{4}{2008.2010}\)

                      \(=2.\frac{2}{2.4}+2.\frac{2}{4.6}+2.\frac{2}{6.8}+...+2.\frac{2}{2008.2010}\)

                      \(=2\left(\frac{2}{2.4}+\frac{2}{4.6}+\frac{2}{6.8}+..+\frac{2}{2008.2010}\right)\)

                      \(=2\left(\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-\frac{1}{6}+\frac{1}{6}-\frac{1}{8}+...+\frac{1}{2008}-\frac{1}{2010}\right)\)

                      \(=2\left(\frac{1}{2}-\frac{1}{2010}\right)\)

                       \(=2.\frac{1}{2}-2.\frac{1}{2010}\)

                       \(=1-\frac{1}{1005}=\frac{1004}{1005}\)

A=(1.2)(2.2)+(2.2)(3.2)+...+(50.2)(51.2)

A=1.2.4+2.3.4+...+50.51.4

A=4(1.2+2.3+...+50.51)

M= 1.2+2.3+...+50.51

3M=1.2.3+2.3.(4-1)+...+50.51.(52-49)

=1.2.3+2.3.4-1.2.3+...+50.51.52-49.50.51

= 50.51.52

=132600

=> M=44200

=> A=4M=176800

\(A=\frac{1}{2}-\frac{1}{4}+\frac{1}{4}-...+\frac{1}{18}-\frac{1}{20}\)

\(A=\frac{1}{2}-\frac{1}{20}\)

\(A=\frac{10}{20}-\frac{1}{20}\)

\(A=\frac{9}{20}\)