题目应是 1/(x^2+x﹚+1/(x^2+3x+2﹚+1/(x^2+5x+6﹚+1/(x^2+7x+12﹚=4/21
1/x(x+1﹚+1/(x+1)(x+2)+1/(x+2)(x+3)+1/(x+3)(x+4)=4/21
1/x -1/(x+1﹚+1/(x+1) -1/(x+2)+1/(x+2)-1/(x+3)+1/(x+3) -1/(x+4)=4/21
1/x -1/(x+4)=4/21
4/x(x+4)=4/21
x^2+4x-21=0
(x-3)(x+7)=0
x=3 x=-7
经检验x=3或x=-7是方程的根