f3數....(2)

(x+y)-1 / [x-1+y-1]-1
= ( x-1 + y-1 ) / ( x + y )
= [ ( 1 / x ) + ( 1 / y ) ] / ( x + y )
= [ ( y + x ) / xy ] / ( x + y ) {通分}
= [ ( x + y ) / xy ] / ( x + y )
= 1 / xy

2007-09-16 00:56:59 補充：
指數法則:( a是正數 )a^-1 = 1 / aa^(m+n) = a^m a^n a^(m-n) = a^m / a^n

(x+y)^(-1)
_________________
[x^(-1)+y^(-1)]^(-1)

=(x+y)^(-1)/(1/x+1/y)^(-1)
=(x+y)^(-1)/[xy/(x+y)]^(-1)
=1/xy