Finding an area of a triangle formed by three points

Click For Summary
SUMMARY

The discussion focuses on calculating the area of a triangle formed by three points in 3D space: P(3, 0, 3), Q(−2, 1, 5), and R(6, 2, 7). The correct method involves finding a nonzero vector orthogonal to the plane through these points using the cross product of vectors PR and PQ. The final area of the triangle is determined to be A = \frac{13\sqrt{5}}{2}, correcting an earlier miscalculation of A = \frac{7}{2}\sqrt{5} due to an error in vector PQ.

PREREQUISITES
  • Understanding of vector operations, specifically cross products
  • Familiarity with 3D coordinate geometry
  • Knowledge of calculating magnitudes of vectors
  • Ability to apply geometric formulas for area calculation
NEXT STEPS
  • Study vector cross product calculations in 3D geometry
  • Learn about the geometric interpretation of vectors and planes
  • Explore advanced area calculations for polygons in 3D space
  • Investigate tools for visualizing 3D geometric figures and their properties
USEFUL FOR

Students studying geometry, particularly those focused on vector mathematics and 3D spatial reasoning, as well as educators seeking to clarify concepts related to area calculations in three-dimensional contexts.

CrosisBH
Messages
27
Reaction score
4

Homework Statement


P(3, 0, 3), Q(−2, 1, 5), R(6, 2, 7)
(a) Find a nonzero vector orthogonal to the plane through the points P, Q,
and R.

(b) Find the area of the triangle PQR

Homework Equations


v1124x.gif

A = \frac{1}{2}|\vec{AB}\times\vec{AC}|
Source: http://www.maths.usyd.edu.au/u/MOW/vectors/vectors-11/v-11-7.html

The Attempt at a Solution



The first part was straight forward and my homework marked it right:

P(3,0,3)\\<br /> Q(-2,1,5)\\<br /> R(6,2,7)\\<br /> \vec{PR} = \langle 3,2,4 \rangle\\<br /> \vec{PQ} = \langle 5,1,2 \rangle
To find a vector orthogonal to two vectors, you do a cross product:
\vec{PR} \times \vec {PQ} = \langle 0,14,-7 \rangle
My homework marked this part right for this answer.

Part two:
First taking the magnitude of the cross product:
|\vec{PR} \times \vec {PQ}| = \sqrt{245} = 7\sqrt{5}
Dividing it by two:
A = \frac{7}{2}\sqrt{5}
Which is my final answer, and the website marks it as wrong, but I can't see why it's wrong. The magnitude is right which I triple checked, by my own hands and Wolfram Alpha, and the order of the cross product is irrelevant in this situation.

I found a calculator that calculates the area from three 3D points and it spit out the nonexact answer of 14.534. My answer is approximately 7.826, a little more than half. I can't see my error unless I'm completely calculating this area wrong. Thanks for the help.
 

Attachments

  • v1124x.gif
    v1124x.gif
    2.7 KB · Views: 714
Physics news on Phys.org
##\vec {PQ}## looks wrong.
 
fresh_42 said:
##\vec {PQ}## looks wrong.
AHHH it was that simple. I was successful in finding a orthogonal vector, but it wasn't the cross product of the two supposed vectors from the points.

Correction:
\vec{PQ} = \langle -5, 1, 2 \rangle
Making the normal vector of interest
\vec{PR} \times \vec {PQ} = \langle 0,-26,13 \rangle
|\vec{PR} \times \vec {PQ}| = 13\sqrt{5}
So the answer is
\frac{13\sqrt{5}}{2}

Let it be the simple subtraction that kills me haha. Thank you
 

Similar threads

  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 7 ·
Replies
7
Views
2K
Replies
7
Views
2K
Replies
6
Views
8K
  • · Replies 4 ·
Replies
4
Views
3K
  • · Replies 5 ·
Replies
5
Views
3K
  • · Replies 5 ·
Replies
5
Views
4K
Replies
3
Views
4K
  • · Replies 4 ·
Replies
4
Views
17K