1 - 5/6 + 7/12 - 9/20 + 11/30 - 13/42 + 15/56 - 17/72 + 19/90= 1 - (3+2)/6 + (4+3)/12 - (5+4)/20 + (6+5)/30 - (7+6)/42 + (8+7)/56 - (9+8)/72 + (10+9)/90= 1 - 1/2 - 1/3 + 1/3 + 1/4 - 1/4 - 1/5 + 1/5 + ...