Solve the Cubic Equation: Finding x When x³ = 1/8

Cubic Equations with Fractional Constants

Solve the following problem:

x3=18 x^3=\frac{1}{8}

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Step-by-step video solution

Watch the teacher solve the problem with clear explanations
00:00 Find X
00:03 Extract the cube root
00:11 When finding the root of a fraction, find the root of both numerator and denominator
00:15 Calculate each of the roots
00:19 And this is the solution to the question

Step-by-step written solution

Follow each step carefully to understand the complete solution
1

Understand the problem

Solve the following problem:

x3=18 x^3=\frac{1}{8}

2

Step-by-step solution

Let's solve the given equation:

x3=18 x^3=\frac{1}{8}

We will perform the inverse operation of the cube power (applied to the unknown) on both sides of the equation. This is the cube root operation. We will need to apply several laws of exponents:

a. Definition of root as a power:

an=a1n \sqrt[n]{a}=a^{\frac{1}{n}}

and two laws of exponents:

b. Law of exponents for power of power:

(am)n=amn (a^m)^n=a^{m\cdot n}

c. Law of exponents for power of parentheses:

(ab)n=anbn \big(\frac{a}{b}\big)^n=\frac{a^n}{b^n}

Let's proceed to solve the equation:
x3=18/3x33=183x33=1383(x3)13=183x313=12x=12 x^3=\frac{1}{8} \hspace{8pt}\text{/}\sqrt[3]{\hspace{6pt}}\\ \sqrt[3]{x^3}=\sqrt[3]{ \frac{1}{8}}\\ \sqrt[3]{x^3}= \frac{\sqrt[3]{1}}{\sqrt[3]{ 8}}\\ (x^3)^{\frac{1}{3}}=\frac{1}{\sqrt[3]{ 8}}\\ x^{3\cdot\frac{1}{3}}=\frac{1}{2}\\ \boxed{x=\frac{1}{2}}

In the first step, we applied the cube root to both sides of the equation. Apply the definition of root as a power (a.) on the left side as well as the law of exponents for power of parentheses (c.) on the right side. Note that raising 1 to any power always yields 1. In the next step, we applied the law of exponents for power of power (b.) Remember that raising a number to the power of 1 doesn't change the number,

Furthermore, given that an odd power preserves the sign of the number it's applied to, taking an odd root requires considering only one possible case which matches the sign of the number being rooted (this is unlike taking an even root, which requires considering two possible cases - positive and negative),

Therefore, the correct answer is answer d.

3

Final Answer

x=12 x=\frac{1}{2}

Key Points to Remember

Essential concepts to master this topic
  • Rule: Apply cube root to both sides to solve cubic equations
  • Technique: Simplify 183=12 \sqrt[3]{\frac{1}{8}} = \frac{1}{2} using root properties
  • Check: Verify (12)3=18 (\frac{1}{2})^3 = \frac{1}{8} confirms the solution ✓

Common Mistakes

Avoid these frequent errors
  • Taking square root instead of cube root
    Don't solve x3=18 x^3 = \frac{1}{8} by taking the square root = wrong operation! This gives an incorrect equation type and impossible answers. Always match the root operation to the exponent: cube root for cube equations.

Practice Quiz

Test your knowledge with interactive questions

Solve the following equation:


\( 2x^2-8=x^2+4 \)

FAQ

Everything you need to know about this question

Why do I take the cube root of both sides?

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To isolate x, you need to undo the cube operation. Since cubing and taking the cube root are inverse operations, applying x3 \sqrt[3]{\phantom{x}} to both sides cancels out the exponent of 3.

How do I find the cube root of a fraction?

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Use the property ab3=a3b3 \sqrt[3]{\frac{a}{b}} = \frac{\sqrt[3]{a}}{\sqrt[3]{b}} . So 183=1383=12 \sqrt[3]{\frac{1}{8}} = \frac{\sqrt[3]{1}}{\sqrt[3]{8}} = \frac{1}{2} since 23=8 2^3 = 8 .

Could there be other solutions besides x = 1/2?

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For real numbers, there's only one solution since cube root gives a unique real value. However, in advanced math, cubic equations can have complex solutions too.

What if I get confused between square roots and cube roots?

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Remember: The root type must match the exponent! For x2 x^2 use square root, for x3 x^3 use cube root, and so on.

How can I check my answer quickly?

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Substitute back: (12)3=1323=18 (\frac{1}{2})^3 = \frac{1^3}{2^3} = \frac{1}{8} ✓. If your calculation gives the original right side, you're correct!

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