1/x（x+1）+1/（x+1）（x+2）+...+1/(x+8)(x+9)
=(1/x - 1/x+1) + (1/x+1 - 1/x+2) + ...+ (1/x+8 - 1/x+9)
=1/x - 1/x+9
=9/x(x+9) = 2x+3/x(x+9)
所以9 = 2x+3
x=3