Ta có: \(A=\frac{1}{3}+\frac{1}{15}+\frac{1}{35}+\frac{1}{63}+\frac{1}{99}\)

\(\Leftrightarrow A=\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+\frac{1}{9.11}\)

\(\Rightarrow2A=\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+\frac{2}{9.11}\)

\(\Rightarrow2A=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}\)

\(\Rightarrow2A=1-\frac{1}{11}=\frac{10}{11}\)

\(\Rightarrow A=\frac{10}{11}:2=\frac{5}{11}\)

Vậy \(A=\frac{5}{11}\)

A = \(\frac{1}{3}+\frac{1}{15}+\frac{1}{35}+\frac{1}{63}+\frac{1}{99}\)

A = \(\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+\frac{1}{9.11}\)

A = \(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}\)

A = \(1-\frac{1}{11}\)

A = \(\frac{10}{11}\)

\(\dfrac{1}{3}+\dfrac{13}{15}+\dfrac{33}{35}+\dfrac{61}{63}+\dfrac{97}{99}\)

\(=\left(1-\dfrac{2}{3}\right)+\left(1-\dfrac{2}{15}\right)+\left(1-\dfrac{2}{35}\right)+\left(1-\dfrac{2}{63}\right)+\left(1-\dfrac{2}{99}\right)\)

\(=\left(1+1+1+1+\right)-\left(\dfrac{2}{3}+\dfrac{2}{15}+\dfrac{2}{35}+\dfrac{2}{63}+\dfrac{2}{99}\right)\)

\(=5-\left(\dfrac{2}{1.3}+\dfrac{2}{3.5}+\dfrac{2}{5.7}+\dfrac{2}{7.9}+\dfrac{2}{9.11}\right)\)

\(=5-\left(1-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+...+\dfrac{1}{9}-\dfrac{1}{11}\right)\)

\(=5-\left(1-\dfrac{1}{11}\right)\)

\(=5-\dfrac{10}{11}\)

\(=\dfrac{45}{11}\)

a=lộn

Ta có : \(A=\frac{1}{3}+\frac{1}{15}+\frac{1}{35}+\frac{1}{63}+\frac{1}{99}\)

\(A=\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}+\frac{1}{7.9}+\frac{1}{9.11}\)

\(2A=\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+\frac{2}{7.9}+\frac{2}{9.11}\)

\(2A=1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+\frac{1}{7}-\frac{1}{9}+\frac{1}{9}-\frac{1}{11}\)

\(2A=1-\frac{1}{11}\)

\(2A=\frac{10}{11}\)

\(A=\frac{10}{11}.\frac{1}{2}=\frac{5}{11}\)

Vì x² luôn luôn lơn hơn hoặc bằng 0

Nên x² + 2015 luôn luôn lơn hơn hoặc bằng 2015

Nên x - 2016 = 0

=> x = 2016 (t/m) 

1/1x3 + 1/3x5 + 1/5x7 + 1/7x9 +... + 1/23x25 + 1/25x27 = 

1/2 x (2/1x3 + 2/3x5 + 2/5x7 +..... 2/23x25 +2/25x27 = 

1/2 x (1 - 1/3 + 1/3 - 1/5 + 1/5 - 1/7 + 1/7 - 1/9 +...... + 1/23 - 1/25 + 1/25 - 1/27 = 

1/2 x (1 - 1/27) =

1/2 x 26/27 = 13/27

13/27

Tk mình đi mọi người mình bị âm nè!

ai tk mình mình tk lại cho!!!

( 1/13 + 1/15 + 1/35 + 1/63 + 1/99 ) x X = 2/3

                                                       X = 2/3 : ( 1/13 + 1/15 + 1/35 + 1/63 + 1/99 )

                                                       X = 286/85

k mình đi mình đang bị âm