Question

1/2X5 + 1/3X5 + 1/8X11 + .+1/20X23 =

Answer 1

1/2*5=(1/2-1/5)1/3

1/3*5=(1/2-1/5)1/3

所以原式=1/3[1/2-1/5+1/5-1/8……+1/20-1/23]

=1/3(1/2-1/23)

=1/3(23/46-2/46)

=1/3(21/46)

=7/46