a) \(A=\frac{1}{2}+\frac{1}{6}+\frac{1}{12}+\frac{1}{20}+\frac{1}{30}+\frac{1}{42}+\frac{1}{56}+\frac{1}{72}+\frac{1}{90}\)
b)Cho \(A=\frac{n+2}{n-5}\left(n\inℤ;n\ne5\right)\). Tìm n để \(A\inℤ\)
c) Tìm \(x\inℕ\), biết:
\(\frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{x.\left(x+1\right)}=\frac{6}{7}\)