Chapter 9: Problem 24
Find an equation of the plane that passes through the points \(P, Q,\) and \(R\) $$P\left(\frac{3}{2}, 4,-2\right), \quad Q\left(-\frac{1}{2}, 2,0\right), \quad R\left(-\frac{1}{2}, 0,2\right)$$
Short Answer
Expert verified
The plane equation is: \( x - z = \frac{7}{2} \).
Step by step solution
01
Find two direction vectors
To find the equation of a plane, we first need two direction vectors that lie on the plane. These can be found by: - Vector \( \vec{PQ} = Q - P \): \[ \vec{PQ} = \left(-\frac{1}{2} - \frac{3}{2}, 2 - 4, 0 + 2\right) = (-2, -2, 2) \] - Vector \( \vec{PR} = R - P \): \[ \vec{PR} = \left(-\frac{1}{2} - \frac{3}{2}, 0 - 4, 2 + 2\right) = (-2, -4, 4) \]
02
Compute the cross product of direction vectors
To find the normal vector to the plane, we compute the cross product of vectors \( \vec{PQ} \) and \( \vec{PR} \). The cross product is given by the determinant: \[ \vec{n} = \vec{PQ} \times \vec{PR} = \begin{vmatrix} \mathbf{i} & \mathbf{j} & \mathbf{k} \ -2 & -2 & 2 \ -2 & -4 & 4 \end{vmatrix} \] Calculating this determinant, we get: \[ \vec{n} = (0, 0, 4) \] However, this is incorrect. Re-evaluate to obtain: \[ \vec{n} = (4, 0, -4) \] This was recalculated considering control steps; ensure calculations yield comprehensive elements.
03
Write the plane equation in normal form
The equation of a plane in normal form is given by \( ax + by + cz = d \), where \( (a, b, c) \) is the normal vector. Using point \( P \left( \frac{3}{2}, 4, -2 \right) \), we plug in the coordinates into the equation: \[ 4(x - \frac{3}{2}) + 0(y - 4) - 4(z + 2) = 0 \] Simplify the equation: \[ 4x - 6 - 4z - 8 = 0 \] \[ 4x - 4z = 14 \] \[ x - z = \frac{7}{2} \] This is the equation of the plane in simplified form.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Direction Vectors
To find a plane's equation using three points, the first step involves determining direction vectors. These vectors are constructed from the given points and lie on the plane.
- Vector \( \vec{PQ} \): Start by finding the vector from point \( P \) to point \( Q \). This is simply the difference between their coordinates. For our exercise, \( \vec{PQ} = Q - P = (-2, -2, 2) \).
- Vector \( \vec{PR} \): Similarly, this vector stretches from point \( P \) to point \( R \). Thus, \( \vec{PR} = R - P = (-2, -4, 4) \).
Cross Product
The cross product is an essential operation when dealing with vectors. It helps find a vector perpendicular to two given vectors, and this perpendicular vector becomes the normal vector for the plane.
- For two vectors \( \vec{u} \) and \( \vec{v} \), their cross product \( \vec{u} \times \vec{v} \) is calculated using a determinant.
- In our example, the cross product \( \vec{PQ} \times \vec{PR} \) is calculated to find the normal vector \( \vec{n} = (4, 0, -4) \).
Normal Vector
A normal vector is a vector that is perpendicular to a plane. It is crucial because it helps to define the plane uniquely against space.
- What is it? It's the result of the cross product of the two direction vectors on the plane. In our context, this is \( \vec{n} = (4, 0, -4) \).
- Importance: The normal vector is needed for writing the plane's equation in its standard form. It establishes the orientation of the plane in three-dimensional space.
Normal Form Equation
Finally, connecting everything, the plane's equation in normal form uses the normal vector and a point on the plane.
- Structure: The normal form of the plane equation is \( ax + by + cz = d \), where \( (a, b, c) \) are the components of the normal vector.
- Substitution: Use one of the original points, such as \( P(\frac{3}{2}, 4, -2) \), to find \( d \). Thus, our equation is simplified to \( 4x - 4z = 14 \).
- Final Equation: After simplification, the final form is \( x - z = \frac{7}{2} \).