\(S=\frac{1.3}{3.5}+\frac{2.4}{5.7}+\frac{3.5}{7.9}+...+\frac{\left(n-1\right)\left(n+1\right)}{\left(2n-1\right)\left(2n+1\right)}+...+\frac{1002.1004}{2005.2007}\)

\(\Rightarrow S=\frac{\left(2-1\right)\left(2+1\right)}{\left(2.2-1\right)\left(2.2+1\right)}+\frac{\left(3-1\right)\left(3+1\right)}{\left(3.2-1\right)\left(3.2+1\right)}+...+\frac{\left(n-1\right)\left(n+1\right)}{\left(2n-1\right)\left(2n+1\right)}\)

\(+..+\frac{\left(1003-1\right)\left(1003+1\right)}{\left(1003.2-1\right)\left(1003.2+1\right)}\)

\(\Rightarrow S=\frac{1}{4}-\frac{3}{8}\left(\frac{1}{2.2-1}-\frac{1}{2.2+1}\right)+\frac{1}{4}-\frac{3}{8}\left(\frac{1}{3.2-1}-\frac{1}{3.2+1}\right)+...\)

\(+\frac{1}{4}-\frac{3}{8}\left(\frac{1}{2n-1}-\frac{1}{2n+1}\right)+...+\frac{1}{4}-\frac{3}{8}\left(\frac{1}{1003.2-1}-\frac{1}{1003.2+1}\right)\)

\(\Rightarrow S=1002.\frac{1}{4}-1002.\frac{3}{8}\left(\frac{1}{2.2-1}-\frac{1}{2.2+1}+\frac{1}{3.2-1}-...-\frac{1}{1003.2+1}\right)\)

\(\Rightarrow S=\frac{501}{2}-\frac{1503}{4}\left(\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{2005}-\frac{1}{2007}\right)\)

\(\Rightarrow S=\frac{501}{2}-\frac{1503}{4}\left(\frac{1}{3}-\frac{1}{2007}\right)\)

\(\Rightarrow S=\frac{501}{2}-\frac{1503}{4}.\frac{668}{2007}\)

\(\Rightarrow S=\frac{501}{2}-\frac{27889}{223}\)

\(\Rightarrow S=125,4372197\)

\(\)

thx  you

1.3+3.5+5.7+...+(2n+1).(2n+3)=(2n+1).(2n+2).(2n+3).(2n+4)

(2n+2)(2n+2)(2n+3)(2n+4):12]+(n+1)

bn lên ngạng hoặc và xem câu hỏi tương tự nha!

Nhớ k mk đấy nha!

thanks nhìu!

OK..OK..OK

\(C=\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}+...+\frac{1}{\left(2n-1\right)\left(2n+1\right)}\)

\(2C=\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{\left(2n-1\right)\left(2n+1\right)}\)

Ta có : 

\(\frac{2}{1.3}=1-\frac{1}{3}\)

\(\frac{2}{3.5}=\frac{1}{3}-\frac{1}{5}\)

...............................

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

\(\Rightarrow2C=1-\frac{1}{2n+1}=\frac{2n}{2n+1}\)

\(\Rightarrow C=\frac{n}{2n+1}\)

BẠN CÓ THỂ VÀO CÂU HỎI TƯƠNG TỰ .

a) Đặt B= 1/1.3 + 1/3.5 + 1/5.7 + .....+ 1/19.21

Ta có: 2B= 2/1.3 + 2/3.5 + 2/5.7 + ....+ 2/19.21

= 1- 1/3 + 1/3-1/5 + 1/5-1/7 +....+ 1/19-1/21

= 1-1/21 = 20/21

=> B= 20/21 : 2 => B= 10/21

b) Như trên, ta có: 2A= 1- (1/2n + 1) => A=( 1-1/2n+1).1/2

=> A= 1/2- 1/2n+1

=> A< 1/2 ( đpcm )

\(I=\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}+...+\frac{1}{\left(2n+1\right).\left(2n+3\right)}\)

\(\Rightarrow I=\frac{1}{2}\left(\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{\left(2n+1\right).\left(2n+3\right)}\right)\)

\(\Rightarrow I=\frac{1}{2}\left(1-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{2n+1}-\frac{1}{2n+3}\right)\)

\(\Rightarrow I=\frac{1}{2}\left(1-\frac{1}{2n+3}\right)\)

\(\Rightarrow I=\frac{1}{2}.\frac{2n+2}{2n+3}\)

\(\Rightarrow I=\frac{n+1}{2n+3}\)

\(I=\frac{1}{1.3}+\frac{1}{3.5}+\frac{1}{5.7}+...+\frac{1}{\left(2n+1\right)\left(2n+3\right)}=\frac{1}{2}.\left(\frac{2}{1.3}+\frac{2}{3.5}+\frac{2}{5.7}+...+\frac{2}{\left(2n+1\right)\left(2n+3\right)}\right)\)

\(=\frac{1}{1}-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+\frac{1}{5}-\frac{1}{7}+...+\frac{1}{2n+1}-\frac{1}{2n+3}=\frac{1}{1}-\frac{1}{2n+3}\)

\(=\frac{2n+3}{2n+3}-\frac{1}{2n+3}=\frac{2n+2}{2n+3}\)

thanks kiệt

tớ làm câu b thôi, câu a nhân 1/2 lên là đc 

\(A=\frac{1}{2}.\left[\left(\frac{2}{1.3}+\frac{2}{3.5}+...+\frac{2}{\left(2n-1\right).\left(2n+1\right)}\right)\right]\)

\(A=\frac{1}{2}.\left(\frac{1}{1}-\frac{1}{3}+\frac{1}{3}-\frac{1}{5}+...+\frac{1}{2.n-1}-\frac{1}{2n+1}\right)\)

\(A=\frac{1}{2}.\left(1-\frac{1}{2n+1}\right)=\frac{1}{2}-\frac{1}{2.\left(2n+1\right)}< \frac{1}{2}\)

p/s: lưu ý không có dấu "=" đâu nhé vì \(\frac{1}{2.\left(2n+1\right)}>0\left(n\text{ thuộc }N\right)\)

CM: \(\dfrac{1}{1.3}\) + \(\dfrac{1}{3.5}\) + \(\dfrac{1}{5.7}\)+...+\(\dfrac{1}{\left(2n+1\right)\left(2n+3\right)}\) = \(\dfrac{n+1}{2n+1}\)

Ta có:

VT = \(\dfrac{1}{2}\) \(\times\) ( \(\dfrac{2}{1.3}\) + \(\dfrac{2}{3.5}\) + \(\dfrac{2}{5.7}\)+....+\(\dfrac{2}{\left(2n+1\right)\left(2n+3\right)}\))

VT = \(\dfrac{1}{2}\) \(\times\) (\(\dfrac{1}{1}\) - \(\dfrac{1}{3}\) + \(\dfrac{1}{3}\) - \(\dfrac{1}{5}\) +  \(\dfrac{1}{5}\) - \(\dfrac{1}{7}\)+....+ \(\dfrac{1}{2n+1}\) - \(\dfrac{1}{2n+3}\))

VT = \(\dfrac{1}{2}\) \(\times\) (\(\dfrac{1}{1}\) - \(\dfrac{1}{2n+3}\) )

VT = \(\dfrac{1}{2}\) \(\times\)( \(\dfrac{2n+3}{2n+3}\) - \(\dfrac{1}{2n+3}\))

VT = \(\dfrac{1}{2}\) \(\times\) \(\dfrac{2n+2}{2n+3}\)

VT = \(\dfrac{1}{2}\)  \(\times\)\(\dfrac{2\times\left(n+1\right)}{2n+3}\)

VT = \(\dfrac{n+1}{2n+3}\)  = VP (đpcm)

\(\dfrac{1}{1.3}+\dfrac{1}{3.5}+\dfrac{1}{5.7}+....+\dfrac{1}{\left(2n-1\right)\left(2n+1\right)}\)

\(=\dfrac{1}{2}\left(\dfrac{2}{1.3}+\dfrac{2}{3.5}+\dfrac{2}{5.7}+....+\dfrac{2}{\left(2n-1\right)\left(2n+1\right)}\right)\)

=\(\dfrac{1}{2}\left(1-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{5}-\dfrac{1}{7}+....+\dfrac{1}{2n-1}-\dfrac{1}{2n+1}\right)\)

=\(\dfrac{1}{2}\left(1-\dfrac{1}{2n+1}\right)\)

=\(\dfrac{1}{2}-\dfrac{1}{4n+2}< \dfrac{1}{2}\)

đặt A=\(\dfrac{1}{1.3}+\dfrac{1}{3.5}+\dfrac{1}{5.7}+....+\dfrac{1}{\left(2n-1\right)\left(2n+1\right)}\)

=> 2A=\(\dfrac{2}{1.3}+\dfrac{2}{3.5}+\dfrac{2}{5.7}+......+\dfrac{2}{\left(2n-1\right)\left(2n+1\right)}\)

<=> 2A=\(1-\dfrac{1}{3}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{7}+.....+\dfrac{1}{2n-2}-\dfrac{1}{2n+1}\)

<=>2A=\(1-\dfrac{1}{2n+1}\)

<=> A=\(\left(1-\dfrac{1}{2n+1}\right)\)\(.\dfrac{1}{2}\)

<=> A=\(\dfrac{1}{2}-\dfrac{1}{2\left(2n+1\right)}\)

=>\(A< \dfrac{1}{2}\) (đpcm)