Show that the equation f(x) = 2x3 + 3x + 6 = 0 has at least one solution; find an interval of length which must contain a solution.

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ISBN:9780470458365
Author:Erwin Kreyszig
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Chapter2: Second-order Linear Odes
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I am still confused on why they used the fraction -3/2 and -1.

Can you explain further?

THank you!

### Question 8

Show that the equation \( f(x) = 2x^3 + 3x + 6 = 0 \) has at least one solution; find an interval of length \(\frac{1}{2}\) which must contain a solution.
Transcribed Image Text:### Question 8 Show that the equation \( f(x) = 2x^3 + 3x + 6 = 0 \) has at least one solution; find an interval of length \(\frac{1}{2}\) which must contain a solution.
### Finding Roots Using the Intermediate Value Theorem

To determine where the function \( f(x) \) has roots, let’s look at the calculations step by step.

Consider the function \( f(x) = 2x^3 + 3x + 6 \).

1. Evaluate \( f(x) \) at \( x = -1 \):

\[ f(-1) = 2(-1)^3 + 3(-1) + 6 \]
\[ f(-1) = 2(-1) + (-3) + 6 \]
\[ f(-1) = -2 - 3 + 6 \]
\[ f(-1) = 1 \]

2. Evaluate \( f(x) \) at \( x = -\frac{3}{2} \):

\[ f\left( -\frac{3}{2} \right) = 2 \left( -\frac{3}{2} \right)^3 + 3 \left( -\frac{3}{2} \right) + 6 \]
\[ f\left( -\frac{3}{2} \right) = 2 \left( -\frac{27}{8} \right) + 3 \left( -\frac{3}{2} \right) + 6 \]
\[ f\left( -\frac{3}{2} \right) = 2 \left( -\frac{27}{8} \right) + \left( -\frac{9}{2} \right) + 6 \]
\[ f\left( -\frac{3}{2} \right) = -\frac{54}{8} - \frac{36}{8} + \frac{48}{8} \]
\[ f\left( -\frac{3}{2} \right) = - \frac{39}{4} \]

Since,

\[ f(-1) \cdot f\left( -\frac{3}{2} \right) < 0 \]

Hence, there exists some \( c \) in the interval \( (-\frac{3}{2}, -1) \) such that \( f(c) = 0 \).

This implies that \( f(x) \) has at least one root in the interval \( \left( -\frac{3}{2}, -1 \right)
Transcribed Image Text:### Finding Roots Using the Intermediate Value Theorem To determine where the function \( f(x) \) has roots, let’s look at the calculations step by step. Consider the function \( f(x) = 2x^3 + 3x + 6 \). 1. Evaluate \( f(x) \) at \( x = -1 \): \[ f(-1) = 2(-1)^3 + 3(-1) + 6 \] \[ f(-1) = 2(-1) + (-3) + 6 \] \[ f(-1) = -2 - 3 + 6 \] \[ f(-1) = 1 \] 2. Evaluate \( f(x) \) at \( x = -\frac{3}{2} \): \[ f\left( -\frac{3}{2} \right) = 2 \left( -\frac{3}{2} \right)^3 + 3 \left( -\frac{3}{2} \right) + 6 \] \[ f\left( -\frac{3}{2} \right) = 2 \left( -\frac{27}{8} \right) + 3 \left( -\frac{3}{2} \right) + 6 \] \[ f\left( -\frac{3}{2} \right) = 2 \left( -\frac{27}{8} \right) + \left( -\frac{9}{2} \right) + 6 \] \[ f\left( -\frac{3}{2} \right) = -\frac{54}{8} - \frac{36}{8} + \frac{48}{8} \] \[ f\left( -\frac{3}{2} \right) = - \frac{39}{4} \] Since, \[ f(-1) \cdot f\left( -\frac{3}{2} \right) < 0 \] Hence, there exists some \( c \) in the interval \( (-\frac{3}{2}, -1) \) such that \( f(c) = 0 \). This implies that \( f(x) \) has at least one root in the interval \( \left( -\frac{3}{2}, -1 \right)
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