Answer:
(a) (0, 3, -1)
(b) (11/2)√10 ≈ 17.3925
Explanation:
Given the points P(1, 0, 1), Q(-2, 1, 4), R(6, 2, 7), you want a normal vector to the plane containing them, and the area of the triangle they form.
Cross product
The cross product of two vectors is orthogonal to both. Its magnitude is ...
|PQ × PR| = |PQ|·|PR|·sin(θ) . . . . where θ is the angle between PQ and PR
The area of triangle PQR can be found from the side lengths PQ and PR as ...
A = 1/2·PQ·PR·sin(θ)
where θ is the angle between the sides.
This means the area of the triangle is half the magnitude of the cross product of two vectors that are its sides.
(a) Orthogonal vector
The attachment shows the cross product of vectors PQ and PR is (0, 33, -11). The components of this vector have a common factor of 11, so we can reduce it to (0, 3, -1).
A normal vector plane PQR is (0, 3, -1).
(b) Area
The area of the triangle is ...
A = 1/2√(0² +33² +(-11)²) = 1/2(11√10)
The area of triangle PQR is (11/2)√10 ≈ 17.3925 square units.
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Additional comment
The area figure can be confirmed by finding the triangle side lengths using the distance formula, then Heron's formula for area from side lengths. The arithmetic is messy, but the result is the same.
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