Math Problem Statement

Sketch the curve for r = 3 + 2cos(θ) and find the area it encloses.

Solution

The given problem involves sketching the polar curve defined by r=3+2cos(θ)r = 3 + 2\cos(\theta), and then finding the area enclosed by this curve.

Step 1: Understanding the Polar Equation

The polar equation given is: r=3+2cos(θ).r = 3 + 2\cos(\theta). This type of equation represents a limaçon with an inner loop, as the coefficient in front of the cosine term (2) is smaller than the constant term (3).

Step 2: Plotting the Curve

The curve exhibits symmetry about the polar axis (xx-axis), as cos(θ)\cos(\theta) is an even function. As θ\theta varies from 00 to 2π2\pi, rr traces out the limaçon.

Step 3: Calculating the Enclosed Area

The area AA enclosed by a polar curve r(θ)r(\theta) from θ=a\theta = a to θ=b\theta = b is given by: A=12abr2dθ.A = \frac{1}{2} \int_a^b r^2 \, d\theta. For a complete loop, we set a=0a = 0 and b=2πb = 2\pi, thus: A=1202π(3+2cos(θ))2dθ.A = \frac{1}{2} \int_0^{2\pi} (3 + 2\cos(\theta))^2 \, d\theta.

Step 4: Expanding (3+2cos(θ))2(3 + 2\cos(\theta))^2

Expanding the expression: (3+2cos(θ))2=9+12cos(θ)+4cos2(θ).(3 + 2\cos(\theta))^2 = 9 + 12\cos(\theta) + 4\cos^2(\theta). We can integrate each term separately: A=1202π(9+12cos(θ)+4cos2(θ))dθ.A = \frac{1}{2} \int_0^{2\pi} \left(9 + 12\cos(\theta) + 4\cos^2(\theta)\right) \, d\theta.

Step 5: Integrating Each Term

  1. 02π9dθ=9(2π)=18π\int_0^{2\pi} 9 \, d\theta = 9(2\pi) = 18\pi.
  2. 02π12cos(θ)dθ=0\int_0^{2\pi} 12\cos(\theta) \, d\theta = 0 (since the integral of cos(θ)\cos(\theta) over a full period is zero).
  3. For 02π4cos2(θ)dθ\int_0^{2\pi} 4\cos^2(\theta) \, d\theta, use the identity cos2(θ)=1+cos(2θ)2\cos^2(\theta) = \frac{1 + \cos(2\theta)}{2}: 02π4cos2(θ)dθ=402π1+cos(2θ)2dθ=202π1dθ+202πcos(2θ)dθ.\int_0^{2\pi} 4\cos^2(\theta) \, d\theta = 4 \int_0^{2\pi} \frac{1 + \cos(2\theta)}{2} \, d\theta = 2 \int_0^{2\pi} 1 \, d\theta + 2 \int_0^{2\pi} \cos(2\theta) \, d\theta. This simplifies to: 2(2π)+0=4π.2(2\pi) + 0 = 4\pi.

Step 6: Adding the Results

Thus, the total area is: A=12(18π+4π)=12×22π=11π.A = \frac{1}{2} (18\pi + 4\pi) = \frac{1}{2} \times 22\pi = 11\pi.

The area enclosed by the curve is 11π11\pi square units.

Do you have any questions or would you like further details?

Here are five related questions to consider:

  1. What happens to the area if the equation changes to r=3+4cos(θ)r = 3 + 4\cos(\theta)?
  2. How would you find the area enclosed by a polar curve if the integration bounds are not from 00 to 2π2\pi?
  3. What is the significance of the coefficients in the equation r=a+bcos(θ)r = a + b\cos(\theta)?
  4. How do different trigonometric identities simplify polar integrals?
  5. Can the equation r=3+2cos(θ)r = 3 + 2\cos(\theta) be converted into Cartesian coordinates?

Tip: When integrating polar equations, remember that rr is squared in the area formula. This accounts for the radial distance's effect on the area calculation.

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Math Problem Analysis

Mathematical Concepts

Polar Coordinates
Limaçon
Area under Curve

Formulas

Area of polar curve: A = (1/2)∫[a to b] r^2 dθ
Cosine double-angle identity: cos^2(θ) = (1 + cos(2θ)) / 2

Theorems

Polar Area Formula

Suitable Grade Level

College-level (Calculus)