Question:medium

The surface area of the plane \(x + 2y + 2z = 12\) cut off by \(x=0, y=0\) and \(x^2+y^2=16\) is

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The formula \( S = \frac{A_{proj}}{|\cos\theta|} \), where \(A_{proj}\) is the area of the projection and \(\theta\) is the angle between the normal to the surface and the normal to the projection plane, is very powerful for finding the area of a planar region. Here, \(\cos\theta = \frac{\vec{n} \cdot \hat{k}}{|\vec{n}||\hat{k}|} = \frac{2}{3}\), so \(S = \frac{4\pi}{2/3} = 6\pi\).
Updated On: Feb 20, 2026
  • 2\(\pi\)
  • 3\(\pi\)
  • 5\(\pi\)
  • 6\(\pi\)
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The Correct Option is D

Solution and Explanation

Step 1: Conceptualization: Determine the surface area of a planar segment. The segment's projection onto the xy-plane is delineated by \(x=0, y=0\) and \(x^2+y^2=16\), identifying it as the first-quadrant sector of the circle \(x^2+y^2=16\).
Step 2: Methodologies: The surface area (S) of a surface \(z = f(x, y)\) over a region R in the xy-plane is computed using \( S = \iint_R \sqrt{1 + \left(\frac{\partial z}{\partial x}\right)^2 + \left(\frac{\partial z}{\partial y}\right)^2} \, dA \). Alternatively, if \( \vec{n} \) is the normal vector to the plane and \( \hat{k} \) is the unit normal to the xy-plane, the surface area is \( S = \frac{A_{xy}}{|\vec{n} \cdot \hat{k}|} \), where \(A_{xy}\) is the projected area.
Step 3: Execution:
Method 1: Integral Calculation
From the plane equation \(x + 2y + 2z = 12\), derive \( z = 6 - \frac{1}{2}x - y \). The partial derivatives are \( \frac{\partial z}{\partial x} = -\frac{1}{2} \) and \( \frac{\partial z}{\partial y} = -1 \). The surface area integrand is \( \sqrt{1 + \left(-\frac{1}{2}\right)^2 + (-1)^2} = \sqrt{1 + \frac{1}{4} + 1} = \frac{3}{2} \). The surface area integral becomes \( S = \iint_R \frac{3}{2} \, dA = \frac{3}{2} \times (\text{Area of R}) \). The region R is a quarter circle of radius 4, so its area is \( \text{Area of R} = \frac{1}{4} (\pi (4)^2) = 4\pi \). Consequently, \( S = \frac{3}{2} \times (4\pi) = 6\pi \).
Method 2: Projection Formula
The normal vector to the plane \(x + 2y + 2z = 12\) is \( \vec{n} = \langle 1, 2, 2 \rangle \). The projected area \(A_{xy}\) is \(4\pi\). The dot product with the xy-plane unit normal \( \hat{k} = \langle 0, 0, 1 \rangle \) is \( |\vec{n} \cdot \hat{k}| = |\langle 1, 2, 2 \rangle \cdot \langle 0, 0, 1 \rangle| = 2 \). The magnitude of the normal vector is \( |\vec{n}| = \sqrt{1^2+2^2+2^2} = 3 \). Using the formula \( S = \frac{|\vec{n}|}{|\vec{n} \cdot \hat{k}|} A_{xy} \), we get \( S = \frac{3}{2} \times (4\pi) = 6\pi \).
Step 4: Conclusion: The calculated surface area is \(6\pi\).
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