Chapter 11: Problem 42
Solve. \(6 x^{2 / 3}-25 x^{1 / 3}-25=0\)
Short Answer
Expert verified
The solutions are \( x = 125 \) and \( x = -\frac{125}{216} \).
Step by step solution
01
Substitution
To solve the equation \(6x^{2/3} - 25x^{1/3} - 25 = 0\), let's make a substitution to simplify it. Let \( y = x^{1/3} \). Therefore, \( y^2 = x^{2/3} \). Substitute these into the equation: \(6y^2 - 25y - 25 = 0\). Now, we will solve this quadratic in terms of \( y \).
02
Solve the Quadratic Equation
The quadratic equation \( 6y^2 - 25y - 25 = 0 \) is in standard form. To solve it, use the quadratic formula \[y = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a}\]where \( a = 6, b = -25, \text{ and } c = -25.\)Plug these values into the formula:\[ y = \frac{-(-25) \pm \sqrt{(-25)^2 - 4 \cdot 6 \cdot (-25)}}{2 \cdot 6} \]\[ y = \frac{25 \pm \sqrt{625 + 600}}{12} \]\[ y = \frac{25 \pm \sqrt{1225}}{12} \]\[ y = \frac{25 \pm 35}{12} \]This gives two potential solutions for \( y \):1. \( y = \frac{60}{12} = 5 \)2. \( y = \frac{-10}{12} = -\frac{5}{6} \).
03
Back-Substitution
Recall that \( y = x^{1/3} \). For each potential value of \( y \), solve for \( x \).1. If \( y = 5 \), then \( x^{1/3} = 5 \), so \( x = 5^3 = 125 \).2. If \( y = -\frac{5}{6} \), then \( x^{1/3} = -\frac{5}{6} \), so \( x = \left( -\frac{5}{6} \right)^3 = -\frac{125}{216} \).
04
Check Solutions
Verify both solutions by substituting them back into the original equation.1. For \( x = 125 \), check if \( 6 \cdot (125)^{2/3} - 25 \cdot (125)^{1/3} - 25 = 0 \). - Compute: \( 125^{1/3} = 5 \), so the equation becomes \( 6 \cdot 5^2 - 25 \cdot 5 - 25 = 0 \). Simplifies to \( 150 - 125 - 25 = 0 \). True.2. For \( x = -\frac{125}{216} \), check if it satisfies the equation: - Compute: \( \left(-\frac{125}{216}\right)^{1/3} = -\frac{5}{6} \), then \( 6 \cdot \left( -\frac{5}{6} \right)^2 - 25 \cdot \left( -\frac{5}{6} \right) - 25 = 0 \). - Simplify: \( 6 \cdot \frac{25}{36} + \frac{125}{6} - 25 = 0 \). This evaluates to \( 0 \), confirming the solution.Both solutions are correct.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Substitution Method
The substitution method is a powerful algebraic technique for simplifying complex equations, making them easier to solve. It involves replacing a part of the original equation with a simpler variable to transform it into a more familiar form. For instance, in the original equation \(6x^{2/3} - 25x^{1/3} - 25 = 0\), we make the substitution \(y = x^{1/3}\). This dramatically simplifies the expression, transforming it into \(6y^2 - 25y - 25 = 0\), a quadratic equation.
- Replace a complex expression with a single variable.
- Result in simpler, more manageable equations.
- Allows for the use of different solving techniques.
Cubic Roots
Cubic roots provide a way to reverse the operation of cubing a number. If you cube a number, raising it to the power of three, you use the cube root to find the original base number. When solving equations like \(x^{2/3}\) or \(x^{1/3}\), cubic roots become essential.Understanding cubic roots is crucial when substituting values, as seen when you use \(y = x^{1/3}\). Here, \(y\) indicates the cube root of \(x\), effectively expressing \(x\) in terms of a simpler form to ease the solution process.
- Cube of \(x\) becomes \(x^3\).
- Cubic root reverses this operation.
- Relevant in problems requiring root solving, such as when transitioning between \(y^2\) and \(x^{2/3}\).
Quadratic Formula
The quadratic formula is a universal tool to solve quadratic equations of the form \(ax^2 + bx + c = 0\). It provides a systematic method to discover the solutions for any quadratic equation in one step using the formula:\[ y = \frac{-b \pm \sqrt{b^2 - 4ac}}{2a} \]In our specific exercise, the equation \(6y^2 - 25y - 25 = 0\) is perfectly suited for this formula. By substituting the constants \(a = 6, b = -25, \text{ and } c = -25\) into the formula, you obtain the solutions:
- \( y = \frac{25 \pm 35}{12} \)
- Leading to \( y = 5 \) or \( y = -\frac{5}{6} \)