1/(1*4)=1/3*(1-1/4)
1/(4*7)=1/3*(1/4-1/7)
原式=1/3×(1-1/4+1/4-1/7+……+1/37-1/40)
=1/3×(1-1/40)
=1/3×39/40
=13/40
1/1X4+1/4x7+1/7X10+.+1/37x40
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