1-2/3+6/5-12/7+.-90/19+110/21用简便算法计算要过程

1-2/3+6/5-12/7+......-90/19+110/21
=1-1-2/1+2/1+3/1-3/1-4/1+……-9/1-10/1+10/1+11/1
=11/1

1-(1*2)/3+(2*3)/5...
1+∑(-1)^n * n(n+1)/(2n+1)
1+∑(-1)^n * [n/1+(n+1)/1]
1-(1/1+2/1)+(2/1+3/1)-(3/1+4/1)...-(9/1+10/1)+(10/1+11/1)
=11/1