Ta có : A = 1/ 2.5 + 1/ 5.8 + 1/ 8.11 + ... + 1/ (3n-1).(3n+2) .
= 1/2 - 1/5 + 1/5 - 1/8 + 1/8 - 1/11 + ... + 1/ 3n-1 - 1/ 3n+2 .
= 1/2 - 1/ 3n+2 .
= 3n + 2 - 2 / 2 .( 3n+2 ) .
= 3n / 2.(3n+2) .
Rút gọn A= \(\frac{1}{2.5}+\frac{1}{5.8}+\frac{1}{8.11}....+\frac{1}{\left(3n+2\right)\left(3n+5\right)}\)
Tính:
\(\left(-1\frac{1}{2}\right).\left(-1\frac{1}{3}\right).....\left(-1\frac{1}{2003}\right).\left(-1\frac{1}{2004}\right)\)
A=\(\frac{15}{2.5}+\frac{15}{5.8}+\frac{15}{8.11}....+\frac{15}{32.35}\)
Tim x biet:\(\left(\frac{1}{2.5}+\frac{1}{5.8}+\frac{1}{8.11}+...+\frac{1}{65.68}\right)x-\frac{7}{34}=\frac{19}{68}\)
Tìm x biết:\(\left(\frac{1}{2.5}+\frac{1}{5.8}+\frac{1}{8.11}+...+\frac{1}{65.68}\right)x-\frac{7}{34}=\frac{19}{68}\)Trả Lời x =
Tìm x biết : \(\left(\frac{1}{2.5}+\frac{1}{5.8}+\frac{1}{8.11}+...+\frac{1}{65.68}\right)x-\frac{7}{34}=\frac{19}{68}\)
Trả lỡi x
TIM X
\(\frac{1}{5.8}+\frac{1}{8.11}+\frac{1}{11.13}+....+\frac{1}{x\left(x+3\right)}\)
tim x biet :
a )\(\frac{1}{5.8}+\frac{1}{8.11}+...+\frac{1}{x\left(x+3\right)}=\frac{101}{1540}\)
b) \(1+\frac{1}{3}+\frac{1}{6}+\frac{1}{10}+...+\frac{1}{x\left(x+1\right).2}=1\frac{1991}{1993}\)
\(S=\frac{1}{2\cdot5}+\frac{1}{5\cdot8}+\frac{1}{8\cdot11}+...+\frac{1}{\left(3n-1\right)\left(3n+2\right)}\) giúp em tìm công thức với ạ
CMR với n thuộc Z, ta có:
\(\left(1+\frac{1}{2}\right).\left(1+\frac{1}{5}\right).\left(1+\frac{1}{9}\right)...\left(1+\frac{2}{n^2+3n}\right)< 3\)
