Question

A= 1/4 + 1/12 + 1/24 + 1/40 +1/60 +1/84           tính nhanh tổng sau

Answer 1

ta có: A = 1/4 + 1/12 + 1/24 + 1/40 + 1/60 + 1/84

            = 2 ( 1/2*4 + 1/4*6 + 1/6*8 + 1/8*10 + 1/10*12 + 1/12*14 )

            = 2 ( 1/2 - 1/4 + 1/4 - 1/6 + 1/6 - 1/8 + 1/8 - 1/10 + 1/10 - 1/12 + 1/12 - 1/14 )

            = 2 ( 1/2 - 1/14 )

            = 6/7

Answer 2

Tính tổng sau:

1/4+1/12+1/24+....+1/1300

Answer 3

ta có: A = 1/4 + 1/12 + 1/24 + 1/40 + 1/60 + 1/84

            = 2 ( 1/2*4 + 1/4*6 + 1/6*8 + 1/8*10 + 1/10*12 + 1/12*14 )

            = 2 ( 1/2 - 1/4 + 1/4 - 1/6 + 1/6 - 1/8 + 1/8 - 1/10 + 1/10 - 1/12 + 1/12 - 1/14 )

            = 2 ( 1/2 - 1/14 )

            = 6/7