12. =3（4x+1）/2(4x+1)=3/2
14. =2(x-7)/2(x-4)(x-7)=1/(x-4)
16. =2x^2/(x-5)
18. =(49x^2-11)/(7x-5)
20. =5x/(2x^2-12x-32)
12.3(4x+1)/2(4x+1)=3/2
14.2(x-7)/2(x-4)(x-7)=1/(x-4)
16.2x*x/(x-5)
18.(49x*x-25)/(7x-5)=7x+5
20.x/2(x+2)(x-8)
12原式=3/2
14 1/(x-4)
16 2x^2/(x-5)
18 7x+5
20 x/[2*(x+2)(x-8)]
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