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Calculate the length of the curve given by x = e2theta costheta and y = e2theta sintheta for 0 theta pi. Calculate the surface area generated when the curve of y = cosx between x = 0 and x = (pi)/(2) rotates about the x-axis.

Mathematics

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2 parts
6.1

52(e2π−1)\frac{\sqrt{5}}{2} (e^{2\pi} - 1)

6.2

π2+πln⁡(1+2)\pi\sqrt{2} + \pi\ln(1+\sqrt{2})

Step-by-step solution

9 steps

Part 6.1· 4 steps

Calculate the length of the curve given by x=e2θcos⁡θx = e^{2\theta} \cos\theta and y=e2θsin⁡θy = e^{2\theta} \sin\theta for 0≤θ≤π0 \leq \theta \leq \pi.

Assumption: Based on typical problem complexity for this type of question, it is assumed that the second function is y=e2θsin⁡θy = e^{2\theta} \sin\theta, as y=e2θcos⁡θy = e^{2\theta} \cos\theta would lead to a significantly more complex integral.

  1. Find the derivatives dxdθ\frac{dx}{d\theta} and dydθ\frac{dy}{d\theta}.

    Given x=e2θcos⁡θx = e^{2\theta} \cos\theta:

    dxdθ=ddθ(e2θcos⁡θ)=(2e2θ)cos⁡θ+e2θ(−sin⁡θ)=e2θ(2cos⁡θ−sin⁡θ)\frac{dx}{d\theta} = \frac{d}{d\theta}(e^{2\theta} \cos\theta) = (2e^{2\theta})\cos\theta + e^{2\theta}(-\sin\theta) = e^{2\theta}(2\cos\theta - \sin\theta)

    Given y=e2θsin⁡θy = e^{2\theta} \sin\theta:

    dydθ=ddθ(e2θsin⁡θ)=(2e2θ)sin⁡θ+e2θ(cos⁡θ)=e2θ(2sin⁡θ+cos⁡θ)\frac{dy}{d\theta} = \frac{d}{d\theta}(e^{2\theta} \sin\theta) = (2e^{2\theta})\sin\theta + e^{2\theta}(\cos\theta) = e^{2\theta}(2\sin\theta + \cos\theta)
  2. Calculate (dxdθ)2+(dydθ)2\left(\frac{dx}{d\theta}\right)^2 + \left(\frac{dy}{d\theta}\right)^2.

    (dxdθ)2=(e2θ(2cos⁡θ−sin⁡θ))2=e4θ(4cos⁡2θ−4sin⁡θcos⁡θ+sin⁡2θ)\left(\frac{dx}{d\theta}\right)^2 = (e^{2\theta}(2\cos\theta - \sin\theta))^2 = e^{4\theta}(4\cos^2\theta - 4\sin\theta\cos\theta + \sin^2\theta) (dydθ)2=(e2θ(2sin⁡θ+cos⁡θ))2=e4θ(4sin⁡2θ+4sin⁡θcos⁡θ+cos⁡2θ)\left(\frac{dy}{d\theta}\right)^2 = (e^{2\theta}(2\sin\theta + \cos\theta))^2 = e^{4\theta}(4\sin^2\theta + 4\sin\theta\cos\theta + \cos^2\theta)

    Summing these:

    (dxdθ)2+(dydθ)2=e4θ(4cos⁡2θ−4sin⁡θcos⁡θ+sin⁡2θ+4sin⁡2θ+4sin⁡θcos⁡θ+cos⁡2θ)\left(\frac{dx}{d\theta}\right)^2 + \left(\frac{dy}{d\theta}\right)^2 = e^{4\theta}(4\cos^2\theta - 4\sin\theta\cos\theta + \sin^2\theta + 4\sin^2\theta + 4\sin\theta\cos\theta + \cos^2\theta) =e4θ(5cos⁡2θ+5sin⁡2θ)=5e4θ(cos⁡2θ+sin⁡2θ)=5e4θ(1)=5e4θ= e^{4\theta}(5\cos^2\theta + 5\sin^2\theta) = 5e^{4\theta}(\cos^2\theta + \sin^2\theta) = 5e^{4\theta}(1) = 5e^{4\theta}
  3. Set up the arc length integral.

    The arc length formula for parametric curves is L=∫αβ(dxdθ)2+(dydθ)2dθL = \int_{\alpha}^{\beta} \sqrt{\left(\frac{dx}{d\theta}\right)^2 + \left(\frac{dy}{d\theta}\right)^2} d\theta.

    L=∫0π5e4θdθ=∫0π5e4θdθ=∫0π5e2θdθL = \int_{0}^{\pi} \sqrt{5e^{4\theta}} d\theta = \int_{0}^{\pi} \sqrt{5} \sqrt{e^{4\theta}} d\theta = \int_{0}^{\pi} \sqrt{5} e^{2\theta} d\theta
  4. Evaluate the integral.

    L=5[12e2θ]0πL = \sqrt{5} \left[ \frac{1}{2}e^{2\theta} \right]_{0}^{\pi} L=52(e2π−e2(0))L = \frac{\sqrt{5}}{2} (e^{2\pi} - e^{2(0)}) L=52(e2π−e0)L = \frac{\sqrt{5}}{2} (e^{2\pi} - e^0) L=52(e2π−1)L = \frac{\sqrt{5}}{2} (e^{2\pi} - 1)

    The length of the curve is 52(e2π−1)\boxed{\frac{\sqrt{5}}{2} (e^{2\pi} - 1)}.

Part 6.2

π2+πln⁡(1+2)\pi\sqrt{2} + \pi\ln(1+\sqrt{2})

Calculate the surface area generated when the curve of y=cos⁡xy = \cos x between x=0x = 0 and x=π2x = \frac{\pi}{2} rotates about the x-axis.

  1. Find the derivative dydx\frac{dy}{dx}.

    Given y=cos⁡xy = \cos x:

    dydx=ddx(cos⁡x)=−sin⁡x\frac{dy}{dx} = \frac{d}{dx}(\cos x) = -\sin x
  2. Calculate 1+(dydx)2\sqrt{1 + \left(\frac{dy}{dx}\right)^2}.

    1+(−sin⁡x)2=1+sin⁡2x\sqrt{1 + (-\sin x)^2} = \sqrt{1 + \sin^2 x}
  3. Set up the surface area integral.

    The surface area of revolution about the x-axis is S=∫ab2πy1+(dydx)2dxS = \int_{a}^{b} 2\pi y \sqrt{1 + \left(\frac{dy}{dx}\right)^2} dx.

    S=∫0π22π(cos⁡x)1+sin⁡2xdxS = \int_{0}^{\frac{\pi}{2}} 2\pi (\cos x) \sqrt{1 + \sin^2 x} dx
  4. Use substitution to evaluate the integral.

    Let u=sin⁡xu = \sin x. Then du=cos⁡x dxdu = \cos x \, dx.
    When x=0x = 0, u=sin⁡(0)=0u = \sin(0) = 0.
    When x=π2x = \frac{\pi}{2}, u=sin⁡(π2)=1u = \sin\left(\frac{\pi}{2}\right) = 1.
    The integral becomes:

    S=2π∫011+u2duS = 2\pi \int_{0}^{1} \sqrt{1 + u^2} du

    This is a standard integral of the form ∫a2+u2du=u2a2+u2+a22ln⁡∣u+a2+u2∣\int \sqrt{a^2 + u^2} du = \frac{u}{2}\sqrt{a^2+u^2} + \frac{a^2}{2}\ln|u+\sqrt{a^2+u^2}|. Here a=1a=1.

    S=2π[u21+u2+12ln⁡∣u+1+u2∣]01S = 2\pi \left[ \frac{u}{2}\sqrt{1+u^2} + \frac{1}{2}\ln|u+\sqrt{1+u^2}| \right]_{0}^{1}
  5. Evaluate the definite integral.

    Evaluate at the upper limit u=1u=1:

    121+12+12ln⁡∣1+1+12∣=122+12ln⁡(1+2)\frac{1}{2}\sqrt{1+1^2} + \frac{1}{2}\ln|1+\sqrt{1+1^2}| = \frac{1}{2}\sqrt{2} + \frac{1}{2}\ln(1+\sqrt{2})

    Evaluate at the lower limit u=0u=0:

    021+02+12ln⁡∣0+1+02∣=0+12ln⁡(1)=0\frac{0}{2}\sqrt{1+0^2} + \frac{1}{2}\ln|0+\sqrt{1+0^2}| = 0 + \frac{1}{2}\ln(1) = 0

    Subtract the lower limit value from the upper limit value:

    S=2π((22+12ln⁡(1+2))−0)S = 2\pi \left( \left(\frac{\sqrt{2}}{2} + \frac{1}{2}\ln(1+\sqrt{2})\right) - 0 \right) S=2π(22+12ln⁡(1+2))S = 2\pi \left( \frac{\sqrt{2}}{2} + \frac{1}{2}\ln(1+\sqrt{2}) \right) S=π2+πln⁡(1+2)S = \pi\sqrt{2} + \pi\ln(1+\sqrt{2})

    The surface area generated is π2+πln⁡(1+2)\boxed{\pi\sqrt{2} + \pi\ln(1+\sqrt{2})}.

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