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Solve the inequality algebraically. \[ x^{3}-2 x^{2}-63 x<0 \] The solution is (Simplify your answer. Type your answer in interval notation. Use integers or fractions for any numbers in the expression.)

Ask by Murray Rodriguez. in Qatar
Dec 07,2024

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Tutor-Verified Answer

Answer

\( x \in (-\infty,-7) \cup (0,9) \)

Solution

Solve the equation \( x^{3}-2 x^{2}-63 x<0 \). Solve the inequality by following steps: - step0: Solve the inequality by testing the values in the interval: \(x^{3}-2x^{2}-63x<0\) - step1: Rewrite the expression: \(x^{3}-2x^{2}-63x=0\) - step2: Factor the expression: \(x\left(x-9\right)\left(x+7\right)=0\) - step3: Separate into possible cases: \(\begin{align}&x=0\\&x-9=0\\&x+7=0\end{align}\) - step4: Solve the equation: \(\begin{align}&x=0\\&x=9\\&x=-7\end{align}\) - step5: Determine the test intervals: \(\begin{align}&x<-7\\&-79\end{align}\) - step6: Choose a value: \(\begin{align}&x_{1}=-8\\&x_{2}=-4\\&x_{3}=5\\&x_{4}=10\end{align}\) - step7: Test the chosen value: \(\begin{align}&x<-7\textrm{ }\textrm{is the solution}\\&-79\textrm{ }\textrm{is not a solution}\end{align}\) - step8: The final solution is \(x \in \left(-\infty,-7\right)\cup \left(0,9\right):\) \(x \in \left(-\infty,-7\right)\cup \left(0,9\right)\) The solution to the inequality \(x^{3}-2x^{2}-63x<0\) is \(x \in (-\infty,-7) \cup (0,9)\).

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To solve the inequality \( x^{3}-2 x^{2}-63 x<0 \), we first factor the expression: 1. Factor out \( x \): \[ x(x^2 - 2x - 63) < 0 \] 2. Next, we need to factor the quadratic \( x^2 - 2x - 63 \). We look for two numbers that multiply to \(-63\) and add to \(-2\). The numbers \(-9\) and \(7\) work: \[ x^2 - 2x - 63 = (x - 9)(x + 7) \] 3. So the inequality becomes: \[ x(x - 9)(x + 7) < 0 \] 4. Now we find the critical points by setting the expression equal to zero: \[ x = 0, \quad x - 9 = 0 \Rightarrow x = 9, \quad x + 7 = 0 \Rightarrow x = -7 \] 5. The critical points are \( x = -7, 0, 9 \). We will test intervals defined by these points: \( (-\infty, -7) \), \( (-7, 0) \), \( (0, 9) \), and \( (9, \infty) \). 6. Choose a test point from each interval: - For \( x < -7 \) (e.g., \( x = -8 \)): \[ -8(-8 - 9)(-8 + 7) = -8(-17)(-1) > 0 \] - For \( -7 < x < 0 \) (e.g., \( x = -1 \)): \[ -1(-1 - 9)(-1 + 7) = -1(-10)(6) < 0 \] - For \( 0 < x < 9 \) (e.g., \( x = 1 \)): \[ 1(1 - 9)(1 + 7) = 1(-8)(8) < 0 \] - For \( x > 9 \) (e.g., \( x = 10 \)): \[ 10(10 - 9)(10 + 7) = 10(1)(17) > 0 \] 7. From the tests, the intervals where the inequality \( x(x - 9)(x + 7) < 0 \) holds are \( (-7, 0) \) and \( (0, 9) \). Thus, the solution in interval notation is: \[ \boxed{(-7, 0) \cup (0, 9)} \]

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