Ta có: 1/2=1/1*2 m*m+1=n
Nên 1/1*2 + 1/2*3 + 1/3*4 +... +1/m*m+1
(1-1/2) + (1/2-1/3) + ... (1/m-1/m+1)
Triệt tiêu đi còn1- 1/m+1 =39/40
Suy ra 1/m+1 = 1/40
Vậy m=39
n = 39* (39+1) = 39*40= 1560
Cho A = \(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+...+\frac{1}{n}\)
Biết A = \(\frac{39}{40}\). Tìm n
Cho \(A=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+...+\frac{1}{n}\)
Biết \(A=\frac{39}{40}\)Tìm n
\(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+...+\frac{1}{n}\)
biết A=\(\frac{39}{40}\)
tìm n
Tìm n :
A = \(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+.....+\frac{1}{n}\)
Biết A = \(\frac{39}{40}\)
Tìm A biết : \(A=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}\)
\(\left(1-\frac{1}{2}-\frac{1}{6}-\frac{1}{12}-\frac{1}{20}-\frac{1}{30}\right).x=\frac{11}{6}\)
Tìm x nha mn
\(T=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+...+\frac{1}{9702}+\frac{1}{9900}\)
Tìm T
\(x+\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}=\frac{47}{42}\)
tìm x
\(\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}\)
