1\12+1\20+1\30+1\42+1\56+1\72+1\90
=1/3x4+1/4x5+1/5x6+……+1/9x10
=1/3-1/4+1/4-1/5+1/5-1/6+……+1/9-1/10
=1/3-1/10
=7/30应该是1\30吧。。就是7/30没错