Bài 3 : Tìm x biết a ) ( 2x+1) ^2 – 4 (x+2)^2 =9 b ) 3 (x -1)^2 – 3x ( x-5 ) = 21 c ) ( x -1 ) ( x^2 + x+1 ) – x ( x+2 ) ( x-2 )= 5 d) ( x-1)^3 –

a ) ( 2x+1) ^2 – 4 (x+2)^2 =9

<->4x^2+4x+1-4(x^2+4x+1)=9

<->4x^2+4x+1-4x^2-16x-16=9

<->-12x -15=9

<->-12x=24

<->x=-2

=>S={-2}

b ) 3 (x -1)^2 – 3x ( x-5 ) = 21

<->3(x^2-2x+1)-3x^2+15x=21

<->3x^2-6x+3-3x^2+15x=21

<->9x+3=21

<->9x=18

<->x=2

=>{2}

c ) ( x -1 ) ( x^2 + x+1 ) – x ( x+2 ) ( x-2 )= 5

<->x^3-1-x(x^2-4)=5

<->x^3-1-x^3-4x=5

<->-4x=6

<->x=-3/2

=>S={-3/2}

d) ( x-1)^3 – ( x+3 ) ( x^2- 3x +9 )+ 3(x -2 )(x+2 ) = 2

<->x^3-3x^2+3x-1-(x^3+27)+3(x^2-4)=2

<->x^3-3x^2+3x-1-x^3-27+3x^2-12=2

<->3x=1+27+12+2

<->3x=42

<->x=14

=>S={14}

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