1/(1*2*3)+1/(2*3*4)+...+1/(20*21*22)
=1/2(1/1*2-1/2*3)+1/2(1/2*3-1/3*4)+...+1/2(1/20*21-1/21*22)
=1/2(1/1*2-1/2*3+1/2*3-1/3*4+...+1/20*21-1/21*22)
=1/2(1/1*2-1/21*22)
=1/2*115/231
=115/462
『『『火速』』』初一数学题～1/1×2×3＋1/2×3×4＋1/3×4×5……＋1/20×21×22
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