(3x-4)/(x^2-5x+4)
(1）要使分式有意义
即x^2-5x+4≠0
(x-4)(x-1)≠0
x≠4且x≠1
(2）要使分式无意义
即x^2-5x+4=0
(x-4)(x-1)=0
x=4或x=1
(3）要使分式值为0
即3x-4=0
x=4/3