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