（1）a/2xy=3ax/6x²y
b/3x² =2by/6x²y
(2)1/(x²+6x+9)=1/(x+3)²=(x-3)/[(x+3)²(x-3)]
2/(x²-9)=2/[(x+3)(x-3)]=2(x+3)/[(x+3)²(x-3)]