Toán Lớp 8: Tìm x
1) 2x(x − 3) + 5(x − 3) = 0
2) 2(x + 3) – x^2 – 3x = 0
3) 4(5−2x)^2 −16=0
4) 3x^3 −4x^2−12x+16=0
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Toán Lớp 8: Tìm x
1) 2x(x − 3) + 5(x − 3) = 0
2) 2(x + 3) – x^2 – 3x = 0
3) 4(5−2x)^2 −16=0
4) 3x^3 −4x^2−12x+16=0
Comments ( 1 )
2x.(x-3)+5.(x-3)=0
<=>(2x+5)(x-3)=0
<=>[(2x+5=0),(x-3=0):}
<=>[(2x=-5),(x=3):}
<=>[(x=-5/2),(x=3):}
Vậy x\in{-5/2;3}
2)
2.(x+3)-x^2-3x=0
<=>2.(x+3)-(x^2+3x)=0
<=>2.(x+3)-x.(x+3)=0
<=>(2-x)(x+3)=0
<=>[(2-x=0),(x+3=0):}
<=>[(x=2),(x=-3):}
Vậy x\in{2;-3}
3)
4.(5-2x)^2-16=0
<=>2^2 . (5-2x)^2-4^2=0
<=>[2.(5-2x)]^2-4^2=0
<=>(10-4x)^2-4^2=0
<=>(10-4x-4)(10-4x+4)=0
<=>(6-4x)(14-4x)=0
<=>[(6-4x=0),(14-4x=0):}
<=>[(4x=6),(4x=14):}
<=>[(x=3/2),(x=7/2):}
Vậy x\in{7/2;3/2}
4)
3x^3-4x^2-12x+16=0
<=>3x^3-6x^2+2x^2-4x-8x+16=0
<=>3x^2.(x-2)+2x.(x-2)-8.(x-2)=0
<=>(3x^2+2x-8)(x-2)=0
<=>(3x^2+6x-4x-8)(x-2)=0
<=>[3x.(x+2)-4.(x+2)](x-2)=0
<=>(3x-4)(x+2)(x-2)=0
<=>[(3x-4=0),(x+2=0),(x-2=0):}
<=>[(3x=4),(x=-2),(x=2):}
<=>[(x=4/3),(x=-2),(x=2):}
Vậy x\in{-2;2;4/3}