請1/1x2x3+1/2x3x4+1/3x4x5+1/4x5x6=

請1/1x2x3+1/2x3x4+1/3x4x5+1/4x5x6=

=1/2×(1/1×2-1/2×3+1/2×3-1/3×4+1/3×4-1/4×5+1/4×5-1/5×6)
=1/2×(1/1×2-1/5×6)
=7/30