1/1x4 + 1/4x7 + 1/7x10 +…+ 1/91x94=?

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1/1x4+1/4x7+1/7x10+…+1/91x94=?1/1x4+1/4x7+1/7x10+…+1/91x94=?1/1x4+1/4x7+1/7x10+…+1/91x94=?1/1x4=(1-1

1/1x4 + 1/4x7 + 1/7x10 +…+ 1/91x94=?
1/1x4 + 1/4x7 + 1/7x10 +…+ 1/91x94=?

1/1x4 + 1/4x7 + 1/7x10 +…+ 1/91x94=?
1/1x4=(1-1/4)/3,1/4x7 =(1/4-1/7)/3,1/7x10 =(1/7-1/10)/3,.1/91x94=(1/91-1/94)/3
所以原式=(1/3)*(1-1/4+1/4-1/7+1/7-1/10+...+1/91-1/94)=(1/3)*(1-1/94)=(1/3)*(93/94)=31/94