(1/x+1/y+1/z)^2
=1/x^2+1/y^2+1/z^2+2/xy+x/yz+2/zx
即有:2^2=1+2*(1/xy+1/yz+1/zx)
所以1/xy+1/yz+1/zx=(4-1)/2=3/2