Let r(t) = (sin(t), cos(t), 6 sin(t) + 9 cos(2t)). Find the projection of r(t) onto the xz-plane for-1 ≤ x (Use symbolic notation and fractions where needed.)

Advanced Engineering Mathematics
10th Edition
ISBN:9780470458365
Author:Erwin Kreyszig
Publisher:Erwin Kreyszig
Chapter2: Second-order Linear Odes
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### Problem Statement

Let \(\mathbf{r}(t) = \langle \sin(t), \cos(t), 6 \sin(t) + 9 \cos(2t) \rangle\).

Find the projection of \(\mathbf{r}(t)\) onto the \(xz\)-plane for \(-1 \leq x \leq 1\).

(Use symbolic notation and fractions where needed.)

### Solution

To project the vector function \(\mathbf{r}(t)\) onto the \(xz\)-plane, we essentially ignore the \(y\)-component. The \(xz\)-plane projection of \(\mathbf{r}(t)\) is given by:

\[ \mathbf{r}_{xz}(t) = \langle \sin(t), 6 \sin(t) + 9 \cos(2t) \rangle \]

Expressing \(z\) as a function of \(x\):

Let \( x = \sin(t) \).

We need to find the corresponding \(z\)-component in terms of \(x\):

\[ z = 6 \sin(t) + 9 \cos(2t) \]

Since \( \cos(2t) = 1 - 2 \sin^2(t) \),

we substitute \( \sin(t) = x \):

\[ z(x) = 6x + 9 (1 - 2x^2) \]
\[ z(x) = 6x + 9 - 18x^2 \]
\[ z(x) = -18x^2 + 6x + 9 \]

Therefore, the projection of \(\mathbf{r}(t)\) onto the \(xz\)-plane can be written as:

\[ z(x) = -18x^2 + 6x + 9 \]

This equation represents the relationship between \(z\) and \(x\) on the \(xz\)-plane for \(-1 \leq x \leq 1\).
Transcribed Image Text:### Problem Statement Let \(\mathbf{r}(t) = \langle \sin(t), \cos(t), 6 \sin(t) + 9 \cos(2t) \rangle\). Find the projection of \(\mathbf{r}(t)\) onto the \(xz\)-plane for \(-1 \leq x \leq 1\). (Use symbolic notation and fractions where needed.) ### Solution To project the vector function \(\mathbf{r}(t)\) onto the \(xz\)-plane, we essentially ignore the \(y\)-component. The \(xz\)-plane projection of \(\mathbf{r}(t)\) is given by: \[ \mathbf{r}_{xz}(t) = \langle \sin(t), 6 \sin(t) + 9 \cos(2t) \rangle \] Expressing \(z\) as a function of \(x\): Let \( x = \sin(t) \). We need to find the corresponding \(z\)-component in terms of \(x\): \[ z = 6 \sin(t) + 9 \cos(2t) \] Since \( \cos(2t) = 1 - 2 \sin^2(t) \), we substitute \( \sin(t) = x \): \[ z(x) = 6x + 9 (1 - 2x^2) \] \[ z(x) = 6x + 9 - 18x^2 \] \[ z(x) = -18x^2 + 6x + 9 \] Therefore, the projection of \(\mathbf{r}(t)\) onto the \(xz\)-plane can be written as: \[ z(x) = -18x^2 + 6x + 9 \] This equation represents the relationship between \(z\) and \(x\) on the \(xz\)-plane for \(-1 \leq x \leq 1\).
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