1/1×4+1/4×7+1/7×10+1/10×13……+1/40×43
=(1-1/4+1/4-1/7+1/7-1/10+1/10-1/13+……+1/40-1/43)÷3
=(1-1/43)÷3
=42/43÷3
=14/43