y=(x-1)(x-3)/(x-1)(2x+1)
==>
x不等于1
y=(x-3)/(2x+1)
==>
y不等于-2/3
y=1/2-7/(4x+2)
==>y的值域 (负无穷,-2/3)U(-2/3,1/2)U(1/2,正无穷)