原式=1/2-(1/2-1/3)-(1/3-1/4)-....-(1/9-1/10) =1/10
1/2-1/6-1/12-1/20-……-1/90=1/2-1/(2×3)-1/(3×4)-1/(4×5)-……-1/(9×10)=1/2-(1/2-1/3)-(1/3-1/4)-……-(1/9-1/10)=1/2-1/2+1/3-1/3+1/4-……-1/9+1/10=1/10