计算：1/2×1/3+1/3×1/4+1/4×1/5+…+1/9×1/10

=1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8-1/9+1/9-1/10=1/2-1/10=2/5=0.4