thay f-2 và f3 vào rồi pạn sẽ tìm ra

a) n là 5 nữa

\(\frac{2}{2}.\left(\frac{1}{10}+\frac{1}{15}+\frac{1}{21}+...+\frac{2}{x+\left(x+1\right)}\right)=\frac{12}{25}\)

\(2.\left(\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+...+\frac{1}{x.\left(x+1\right)}\right)=\frac{12}{25}\)

\(2.\left(\frac{1}{4.5}+\frac{1}{5.6}+\frac{1}{6.7}+...+\frac{1}{x.\left(x+1\right)}\right)=\frac{12}{25}\)

\(2.\left(\frac{1}{4}-\frac{1}{5}+\frac{1}{5}-\frac{1}{6}+\frac{1}{6}-\frac{1}{7}+...+\frac{1}{x}-\frac{1}{x+1}\right)=\frac{12}{25}\)

\(2.\left(\frac{1}{4}-\frac{1}{x+1}\right)=\frac{12}{25}\)

\(\frac{1}{2}-\frac{2}{x+1}=\frac{12}{25}\)

\(\frac{2}{x+1}=\frac{1}{2}-\frac{12}{25}\)

\(\frac{2}{x+1}=\frac{1}{50}\)

(x+1).1=50.2

x+1=100

x=100-1

x=99

(!help me!)