1/(x+2)=(y-3)/(y-4)
两边分母分别减去各自分子：
1/(x+2-1)=(y-3)/(y-4-y+3)
1/(x+1)=(y-3)/(-1)
y-3=-1/(x+1)
y=3-1/(x+1)=(3x+3-1)/(x+1)=(3x+2)/(x+1)