1/xy=(1/x-1/y)*(1/y-x)1/(x+4)(y+4)=(1/(x+4)-1/(y+4))*(1/y-x).以此推类提取(1/y-x) 原式=( 1/y-x)*(1/x-1/y+1/(x+4)-1/(y+4).1/（x+4n)-1/(y+4n））因为x=4,y=8所以 原式=(1/4)*( 1/4 -1/8+1/8 -1/12.1/（4+4n) -...