Toán Lớp 8: (x+1)^3 -(x-2)^3 = (3x+1)(3x-1)
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Toán Lớp 8: (x+1)^3 -(x-2)^3 = (3x+1)(3x-1)
Comments ( 2 )
+)(x+1)^3 -(x-2)^3 = (3x+1)(3x-1)
$$\boxed{\text{Giải:}}$$
(x+1)^3 -(x-2)^3 = (3x+1)(3x-1)
<=>x^3+3x^2+3x+1-x^3+6x^2-12x+8=9x^2-1
<=>x^3-x^3+3x^2+6x^2-9x^2+3x-12x=-1-1-8
<=>-9x=-10
<=>x=(-10):(-9)
<=>x=10/9
Vậy ,x=10/9
$ ( x+1)^3 – (x-2)^3 = (3x+1)(3x-1) $
$ ⇒ 9x^2 – 9x+9 = 3x(3x-1) + ( 3x-1 ) $
$ ⇒ 9x^2 – 9x+9 = 9x^2-1 $
$ ⇒ 9x^2 – 9x + 9 – ( 9x^2-1) = 0 $
$ ⇒ 9x^2 – 9x+9 – 9x^2+1=0 $
$ ⇒ -9x + 10 =0 $
$ ⇒ x = \dfrac{10}{9} $