=(-1/1×2)-(1/2×3)+...+(-1/9×10)
=-(1-1/2)-(1/2-1/3)-(1/3-1/4)...-(1/9-1/10)
=-1+1/2-1/2+1/3-1/3+...+1/9-1/9+1/10
=-1+1/10
=-9/10