（1）a1=1/2-[1+(-1/2)]=1/2-1/2=0a2=1/3-[1+(-1/2)][1+(-1)2/3][1+(-1)3/4]=1/3-1/2×4/3×3/4=1/3-1/2=-1/6a3=1/4-[1+(-1/2)][1+(-1)2/3][1+(-1)3/4][1+(-1)4/5][1+(-1)6/6]=1/4-1/2×4/3×3/4×6/5×5/6=1/4-1/2=...