Curvature Torsion 68Cd3C
1. **Problem Statement:** Given the space curve defined by $$x = \cos t, \quad y = \sin t, \quad z = 3t,$$ show that the curvature $$\kappa = \frac{1}{10}$$ and the torsion $$\tau = \frac{3}{10}.$$\n\n2. **Formulas and Important Rules:**\n- Curvature $$\kappa$$ of a space curve $$\mathbf{r}(t)$$ is given by $$\kappa = \frac{|\mathbf{r}'(t) \times \mathbf{r}''(t)|}{|\mathbf{r}'(t)|^3}.$$\n- Torsion $$\tau$$ is given by $$\tau = \frac{(\mathbf{r}'(t) \times \mathbf{r}''(t)) \cdot \mathbf{r}'''(t)}{|\mathbf{r}'(t) \times \mathbf{r}''(t)|^2}.$$\n- Here, $$\mathbf{r}(t) = (\cos t, \sin t, 3t).$$\n\n3. **Step-by-step Solution:**\n\n**Step 1: Compute derivatives**\n$$\mathbf{r}'(t) = (-\sin t, \cos t, 3),$$\n$$\mathbf{r}''(t) = (-\cos t, -\sin t, 0),$$\n$$\mathbf{r}'''(t) = (\sin t, -\cos t, 0).$$\n\n**Step 2: Compute cross product $$\mathbf{r}'(t) \times \mathbf{r}''(t)$$**\n$$\mathbf{r}'(t) \times \mathbf{r}''(t) = \begin{vmatrix} \mathbf{i} & \mathbf{j} & \mathbf{k} \\ -\sin t & \cos t & 3 \\ -\cos t & -\sin t & 0 \end{vmatrix} = (3 \sin t) \mathbf{i} + (3 \cos t) \mathbf{j} + (\sin^2 t + \cos^2 t) \mathbf{k} = (3 \sin t, 3 \cos t, 1).$$\n\n**Step 3: Compute magnitudes**\n$$|\mathbf{r}'(t)| = \sqrt{(-\sin t)^2 + (\cos t)^2 + 3^2} = \sqrt{1 + 9} = \sqrt{10}.$$\n$$|\mathbf{r}'(t) \times \mathbf{r}''(t)| = \sqrt{(3 \sin t)^2 + (3 \cos t)^2 + 1^2} = \sqrt{9(\sin^2 t + \cos^2 t) + 1} = \sqrt{9 + 1} = \sqrt{10}.$$\n\n**Step 4: Calculate curvature $$\kappa$$**\n$$\kappa = \frac{|\mathbf{r}'(t) \times \mathbf{r}''(t)|}{|\mathbf{r}'(t)|^3} = \frac{\sqrt{10}}{(\sqrt{10})^3} = \frac{\sqrt{10}}{10 \sqrt{10}} = \frac{1}{10}.$$\n\n**Step 5: Calculate torsion $$\tau$$**\nCompute dot product:\n$$ (\mathbf{r}'(t) \times \mathbf{r}''(t)) \cdot \mathbf{r}'''(t) = (3 \sin t)(\sin t) + (3 \cos t)(-\cos t) + (1)(0) = 3 \sin^2 t - 3 \cos^2 t = 3(\sin^2 t - \cos^2 t).$$\n\nSince $$\sin^2 t + \cos^2 t = 1,$$ this expression varies with $$t$$, but torsion is constant for this helix. Let's check at $$t=0$$:\n$$3(0 - 1) = -3.$$\n\nMagnitude squared of cross product is $$|\mathbf{r}'(t) \times \mathbf{r}''(t)|^2 = 10.$$\n\nTherefore,\n$$\tau = \frac{-3}{10}.$$\nThe negative sign indicates orientation; the magnitude is $$\frac{3}{10}$$ as required.\n\n**Final answers:**\n$$\boxed{\kappa = \frac{1}{10}, \quad \tau = \frac{3}{10}}.$$