Question

\(\frac{1}{1\times3}+\frac{1}{3\times5}+\frac{1}{5\times7}...+\frac{1}{19\times21}\)= A.Tìm A

Answer 1

\(\frac{1}{1\times3}\) \(+\) \(\frac{1}{3\times5}\) \(+\) \(\frac{1}{5\times7}\) \(+\) \(...\) \(\frac{1}{19\times21}\)

\(=\) \(\frac{943}{1995}\)

Answer 2

A=1-1 phần 2 +1 phần 2 -1 phần 3.... + 1phan19 - 1 phần 21

A= 1- 1 phần 21

A= 20 phần 21

Answer 3

A x 2 = \(\frac{1}{1}\)-\(\frac{1}{1}\)+\(\frac{1}{3}\)-.........- \(\frac{1}{19}\)+\(\frac{1}{21}\)

A x 2 = \(\frac{1}{1}\)- \(\frac{1}{21}\)= \(\frac{20}{21}\)

A= \(\frac{20}{21}\): 2 = \(\frac{10}{21}\)

Answer 4

943/1995

ai k mk thì mk k lại