9的约数有1,3,9
所以
1/9=1/18+1/18
=(1+3)/9×(1+3)
=1/36+1/12
=(1+9)/9×(1+9)
=1/90+1/10
共3种:1/9=1/18+1/18=1/36+1/12=1/90+1/10