Question

1/4x6+1/6x8+...1/78x80 giúp mik nha cảm ơn

Answer 1

Đặt A = \(\frac{1}{4\times6}+\frac{1}{6\times8}+....+\frac{1}{78\times80}\)

    2A = \(\frac{2}{4\times6}+\frac{2}{6\times8}+....+\frac{2}{78\times80}\)

     2A = \(\frac{1}{4}-\frac{1}{6}+\frac{1}{6}-\frac{1}{8}+....+\frac{1}{78}-\frac{1}{80}\)

    2A = \(\frac{1}{4}-\frac{1}{80}=\frac{19}{80}\)

=> A = \(\frac{19}{80}:2=\frac{19}{160}\)