通分 y+3/y*y-6y+9,1/2y+6,y-1/y*y-9

y²-6y+9=(y-3)²
2y+6=2(y+3)
y²-9=(y+3)(y-3)
最简公分母是 2(y+3)(y-3)²
(y+3)/(y²-6y+9)=(y+3)/(y-3)²
=[2(y+3)²]/[2(y+3)(y-3)²]
1/(2y+6)=1/[2(y+3)]
=(y-3)²/[2(y+3)(y-3)²]
(y-1)/(y²-9)=(y-1)/[(y+3)(y-3)]
=[2(y-1)(y-3)]/[2(y+3)(y-3)²]