Đặt \(A=\frac{99}{1}+\frac{98}{2}+\frac{97}{3}+...+\frac{1}{99}\), ta có:
\(A=\frac{99}{1}+\frac{98}{2}+\frac{97}{3}+...+\frac{1}{99}=\frac{100-1}{1}+\frac{100-2}{2}+\frac{100-3}{3}+...+\frac{100-99}{99}\)
\(=100-1+\frac{100}{2}-1+\frac{100}{3}-1+...+\frac{100}{99}-1\)
\(=100+\left(\frac{100}{2}+\frac{100}{3}+...+\frac{100}{99}\right)-\left(1+1+1+...+1\right)\)
\(=100+\left(\frac{100}{2}+\frac{100}{3}+...+\frac{100}{99}\right)-99\)
\(=1+100\left(\frac{1}{2}+\frac{1}{3}+...+\frac{1}{99}\right)\)
\(A=100\left(\frac{1}{2}+\frac{1}{3}+...+\frac{1}{99}+\frac{1}{100}\right)\)
Do đó, \(\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+...+\frac{1}{100}=\frac{1}{100}.A\)
Vậy, \(B=\frac{\frac{1}{100}.A}{A}=\frac{1}{100}\)
