Dễ quá
1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
Đặt A = 1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
A = 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/99.100
A = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + .... + 1/99 - 1/100]
A = 1/1 - 1/100
A = 99/100
Vậy A = 99/100
1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
= 1/1x2 + 1/2x3 + 1/3x4 + 1/4x5 + ... + 1/99x100
= 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + ... + 1/99 - 1/100
= 1 - 1/100
= 99/100
1/1.2+1/2.3+1/3.4+....+1/99.100
1-1/2+1/2-1/3+1/3-1/4+.....+1/99-1/100
1-1/100
99/100
1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
Đặt A = 1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
A = 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/99.100
A = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + .... + 1/99 - 1/100]
A = 1/1 - 1/100
A = 99/100
Vậy A = 99/100
Thấy đúng thì ủng hộ mk nha ^_^
mk ủng hộ ý kiến của Mai, mk cũng định làm thế đó, k cho Mai đi
Ta có : \(\frac{1}{\frac{n-1}{\left(n+1\right)}}=\frac{1}{\left[n\left(n+1\right)\right]}\)(chứng minh bằng cách quy đồng rút gọn vế trái)
\(S=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+...+\frac{1}{9900}\)
\(=\frac{1}{\left(1\times2\right)}+\frac{1}{\left(2\times3\right)}+\frac{1}{\left(3\times4\right)}+...+\frac{1}{\left(99\times100\right)}\)
\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{99}-\frac{1}{100}\)
\(=\frac{99}{100}\)
Đặt A = 1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
A = 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/99.100
A = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + .... + 1/99 - 1/100]
A = 1/1 - 1/100
A = 99/100
Vậy A = 99/100
Đặt A = 1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
A = 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/99.100
A = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + .... + 1/99 - 1/100]
A = 1/1 - 1/100
A = 99/100
Vậy A = 99/100
1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
Đặt A = 1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
A = 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/99.100
A = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + .... + 1/99 - 1/100]
A = 1/1 - 1/100
A = 99/100
Vậy A = 99/100
Thấy đúng thì ủng hộ mk nha ^_^
1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
Đặt A = 1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
A = 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/99.100
A = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + .... + 1/99 - 1/100]
A = 1/1 - 1/100
A = 99/100
Vậy A = 99/100
Thấy đúng thì ủng hộ mk nha ^_^
1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
Đặt A = 1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
A = 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/99.100
A = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + .... + 1/99 - 1/100]
A = 1/1 - 1/100
A = 99/100
Vậy A = 99/100
Thấy đúng thì ủng hộ mk nha ^_^
1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
Đặt A = 1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
A = 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/99.100
A = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + .... + 1/99 - 1/100]
A = 1/1 - 1/100
A = 99/100
Vậy A = 99/100
Thấy đúng thì ủng hộ mk nha ^_^
1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
Đặt A = 1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
A = 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/99.100
A = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + .... + 1/99 - 1/100]
A = 1/1 - 1/100
A = 99/100
Vậy A = 99/100
Thấy đúng thì ủng hộ mk nha ^_^
1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
Đặt A = 1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
A = 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/99.100
A = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + .... + 1/99 - 1/100]
A = 1/1 - 1/100
A = 99/100
Vậy A = 99/100
Thấy đúng thì ủng hộ mk nha ^_^
1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
Đặt A = 1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
A = 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/99.100
A = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + .... + 1/99 - 1/100]
A = 1/1 - 1/100
A = 99/100
Vậy A = 99/100
1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
Đặt A = 1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
A = 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/99.100
A = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + .... + 1/99 - 1/100]
A = 1/1 - 1/100
A = 99/100
Vậy A = 99/100
1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
Đặt A = 1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
A = 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/99.100
A = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + .... + 1/99 - 1/100]
A = 1/1 - 1/100
A = 99/100
Vậy A = 99/100
1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
Đặt A = 1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
A = 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/99.100
A = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + .... + 1/99 - 1/100]
A = 1/1 - 1/100
A = 99/100
Vậy A = 99/100
1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
Đặt A = 1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
A = 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/99.100
A = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + .... + 1/99 - 1/100]
A = 1/1 - 1/100
A = 99/100
Vậy A = 99/100
Thấy đúng thì ủng hộ mk nha ^_^
1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
Đặt A = 1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
A = 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/99.100
A = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + .... + 1/99 - 1/100]
A = 1/1 - 1/100
A = 99/100
Vậy A = 99/100
Thấy đúng thì ủng hộ mk nha ^_^
1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
Đặt A = 1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
A = 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/99.100
A = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + .... + 1/99 - 1/100]
A = 1/1 - 1/100
A = 99/100
Vậy A = 99/100
Thấy đúng thì ủng hộ mk nha ^_^
1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
Đặt A = 1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
A = 1/1.2 + 1/2.3 + 1/3.4 + 1/4.5 + ... + 1/99.100
A = 1/1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + .... + 1/99 - 1/100]
A = 1/1 - 1/100
A = 99/100
Vậy A = 99/100
Thấy đúng thì ủng hộ mk nha ^_^
1/2 + 1/6 + 1/12 + 1/20 + ... + 1/9900
= 1/1x2 + 1/2x3 + 1/3x4 + 1/4x5 + ... + 1/99x100
= 1 - 1/2 + 1/2 - 1/3 + 1/3 - 1/4 + 1/4 - 1/5 + ... + 1/99 - 1/100
= 1 - 1/100
= 99/100