1/2x3十1/3x4十1/4x5十……十1/18

1/2x3十1/3x4十1/4x5十……十1/18x19十1/19x20=?

1个回答

1/(2x3)十1/(3x4)十1/(4x5)十……十1/(18x19)十1/(19x20)
=(1/2-1/3)十(1/3-1/4)十(-1/4-1/5)十……十(1/18-1/19)十(1/19-1/20)
=1/2-1/20
=9/20