x(x-1)分之1+x(x+1)分之1+(x+1)(x+2)分之1+(x+2)(x+3)分之1
=1/(x-1)-1/x+1/x-1/(x+1)+1/(x+1)-1/(x+2)+1/(x+2)-1/(x+3)
=1/(x-1)-1/(x+3)
=4/(x-1)(x+3);
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