a) Gọi ƯCLN(12n+1;30n+2) = d
\(\Rightarrow\begin{cases}12n+1⋮d\\30n+2⋮d\end{cases}\)
\(\Rightarrow\begin{cases}5\left(12n+1\right)⋮d\\2\left(30n+2\right)⋮d\end{cases}\)
\(\Rightarrow\begin{cases}60n+5⋮d\\60n+4⋮d\end{cases}\)
=> ( 60n + 5 ) - ( 60n + 4 ) \(⋮\) d
=> 1 \(⋮\) d
=> d = 1
Vậy \(\frac{12n+1}{30n+2}\) là phân số tối giản
b) Ta có : \(\frac{1}{2^2}< \frac{1}{1.2}\)
\(\frac{1}{3^2}< \frac{1}{2.3}\)
.........
\(\frac{1}{100^2}< \frac{1}{99.100}\)
=> \(\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+...+\frac{1}{100^2}< \frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{99.100}\)
Mà \(\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{99.100}\)
\(=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+...+\frac{1}{99}-\frac{1}{100}\)
\(=1-\frac{1}{100}< 1\)
\(\Rightarrow\frac{1}{2^2}+\frac{1}{3^2}+\frac{1}{4^2}+...+\frac{1}{100^2}< 1\) ( đpcm )
