计算:1/1X3+1/2X4+1/3X5+.+1/18X20=?并说明理由 快加把劲

问题描述:

计算:1/1X3+1/2X4+1/3X5+.+1/18X20=?并说明理由 快加把劲

1/1X3+1/2X4+1/3X5+.+1/18X20=1/1X3+1/3X5+1/2×4+1/4×6.+1/17×19+1/18X20=(1-1/3+1/3-1/5+……+1/17-1/19)/2+(1/2-1/4+1/4-1/6+……+1/18-1/20)/2=(1-1/19)/2+(1/2-1/20)/2=9/19+9/40=531/760...