
1÷(x+1)(x+2)+1÷(x+2)(x+3)+1÷(x+3)(x+4)+1÷(x+4)(x+5)=
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1/[n(n+1)]=1/n-1/(n+1)
原式=1/(x+1)-1/(x+2)+1/(x+2)-1/(x+3)+1/(x+3)-1/(x+4)+1/(x+4)-1/(x+5)=4/[(x+1)(x+5)]
原式=1/(x+1)-1/(x+2)+1/(x+2)-1/(x+3)+1/(x+3)-1/(x+4)+1/(x+4)-1/(x+5)=4/[(x+1)(x+5)]
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