Giải thích các bước giải:
(5+4x)(-x+2)=(5+4x)(7+5x)
⇔(5+4x)(-x+2)-(5+4x)(7+5x)=0
⇔(5+4x)(-x+2-7-5x)=0
⇔(5+4x)(-6x-5)=0
⇔5+4x=0
-6x-5=0
⇔x=-5/4
x=-5/6
Vậy S={-5/4;-5/6}
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(4+x)(x-5)=(3x-8)(x-5)=0
⇔(4+x)(x-5)-(3x-8)(x-5)=0
⇔(x-5)(4+x-3x+8)=0
⇔(x-5)(-2x+12)=0
⇔x-5=0
-2x+12=0
⇔x=5
x=6
Vậy {5;6}