化简：(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+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))=1/21x=3