Ta có:

Xét số a. Ta có a² > (a - 1)(a + 1)

Thật vậy, (a - 1)(a + 1) = a(a + 1) - (a + 1) = a² + a - a - 1 = a² - 1 < a²

Suy ra \(\dfrac{1}{\left(a-1\right)\left(a+1\right)}>\dfrac{1}{a^2}\)

Ta có:

\(\dfrac{1}{2^2}+\dfrac{1}{3^2}+\dfrac{1}{4^2}+...+\dfrac{1}{100^2}\)

\(< \dfrac{1}{1.3}+\dfrac{1}{2.4}+\dfrac{1}{3.5}+...+\dfrac{1}{99.101}\)

\(=\dfrac{1}{2}\left(1-\dfrac{1}{3}+\dfrac{1}{2}-\dfrac{1}{4}+\dfrac{1}{3}-\dfrac{1}{5}+\dfrac{1}{4}-\dfrac{1}{6}+...+\dfrac{1}{99}-\dfrac{1}{101}\right)\)

\(=\dfrac{1}{2}\left(1+\dfrac{1}{2}-\dfrac{1}{100}-\dfrac{1}{101}\right)\)

\(< \dfrac{3}{4}\)

Ko bt có sai chỗ nào ko....

Ta có : \(\frac{1}{2}+\frac{2}{3}+..+\frac{99}{100}\)

= \((1-\frac{1}{2})+(1-\frac{1}{3})+...+(1-\frac{99}{100})\)(100 cặp số )

= \(\left(1+1+1+...+1\right)-\left(\frac{1}{2}+\frac{1}{3}+...+\frac{1}{100}\right)\)(100 số hạng 1)

= \(1\times100-\left(\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+..+\frac{1}{100}\right)\)

= \(100-\left(\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+...+\frac{1}{100}\right)\)

=> 100-(1+1/2+1/3+...+1/100) = 1/2+2/3+3/4+...+99/100

Bạn cố giải cho mình dễ hiểu hơn ko?

  ta có 
1/2^2 < 1/(1.2)= 1-1/2 
1/3^2 <1/(2.3)=1/2-1/3 
1/4^2 <1/(3.4)=1/3-1/4 
...... 
1/100^2 < 1/99-1/100 
cộng vế với vế ta được 1/2^2 +1/3^2+...< 1-1/2+1/2-1/3+....+1/99-1/100=1-1/100 
=> ĐPCM

Ta có: 1/3^2<1/2.3;1/4^2<1/3.4;........

=>1/3^2+1/4^2+1/5^2+......+1/100^2

< 1/2.3+1/3.4+1/4.5+.....+1/99.100

=1/2-1/3+1/3-1/4+1/4-1/5+...+1/99-1/100

=1/2-1/100

=49/100

Mà 49/100<1/2

Nên 1/3^2+1/4^2+1/5^2+......+1/100^2<1/2

Đ ú n g nha......................