1/1乘2乘3+1/2乘3乘4+1/3乘4乘5

题目

1/1乘2乘3+1/2乘3乘4+1/3乘4乘5

答案

=1/2×(1/1×2-1/2×3+1/2×3-1/3×4+1/3×4-1/4×5)
=1/2×(1/1×2-1/4×5)
=9/40