Cubic equation

Algebra Level 1

x 3 3 x 2 3 + 1 = x \sqrt[3]{x^3-3x^2}+1 =x

Solve for the value of x x .

1 3 \frac13 2 5 \frac25 4 5 \frac45 8 3 \frac83

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2 solutions

x 3 3 x 2 3 + 1 = x x 3 3 x 2 3 = x 1 x 3 3 x 2 = x 3 3 x 2 + 3 x 1 3 x 1 = 0 x = 1 3 \sqrt[3]{x^3-3x^2}+1=x\\\sqrt[3]{x^3-3x^2}=x-1\\x^3-3x^2=x^3-3x^2+3x-1\\3x-1=0\rightarrow x=\frac{1}{3}

Jesse Nieminen
Jul 9, 2019

We have x 3 3 x 2 3 + 1 = x . \sqrt[3]{x^3 - 3x^2} + 1 = x. After raising both sides of the equation to the third power it follows that x 3 3 x 2 = ( x 1 ) 3 = x 3 3 x 2 + 3 x 1. x^3 - 3x^2 = \left(x-1\right)^3 = x^3 - 3x^2 + 3x - 1. Now we cancel the like terms and we are left with x = 1 3 . x = \frac{1}{3}.

This tells us that if x x is a solution to the original equation it must be equal to 1 3 \dfrac{1}{3} . Substituting x = 1 3 x = \frac{1}{3} to the original equation yields ( 1 3 ) 3 3 ( 1 3 ) 2 3 + 1 = 1 3 1 27 9 27 3 + 1 = 1 3 8 27 3 + 1 = 1 3 2 3 + 1 = 1 3 1 3 = 1 3 . \begin{aligned} \sqrt[3]{\left(\dfrac{1}{3}\right)^3 - 3\left(\dfrac{1}{3}\right)^2} + 1 &= \dfrac{1}{3} \\ \sqrt[3]{\dfrac{1}{27} - \dfrac{9}{27}} + 1 &= \dfrac{1}{3} \\ \sqrt[3]{-\dfrac{8}{27}} + 1 &= \dfrac{1}{3} \\ -\dfrac{2}{3} + 1 &= \dfrac{1}{3} \\ \dfrac{1}{3} &= \dfrac{1}{3}. \\ \end{aligned}

Which tells us that x = 1 3 x = \frac{1}{3} is indeed a solution to the original equation.

Hence the solution is 1 3 \boxed{\dfrac{1}{3}} .

Here 2nd step is slight wrong..plz correct it.-9/27 is wrong. correct is -1/3

Arun Mahanta - 1 year, 2 months ago

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It isn't wrong. 9 27 -\dfrac{9}{27} is equal to 1 3 -\dfrac{1}{3} .

Jesse Nieminen - 1 year, 2 months ago

-9/27=1/3 usual simplification.

howard merryweather - 9 months, 3 weeks ago

misprint -sign missing

howard merryweather - 9 months, 3 weeks ago

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