Register to reply

Naming a triangle for a vectors question

by mazz1801
Tags: triangle, vector
Share this thread:
mazz1801
#1
Apr10-12, 06:54 AM
P: 23
1. The problem statement, all variables and given/known data

Triangle ABC has A (−1, 3,−3), B (2, 4, 6) and C (3, 0,−5). Use the scalar
product to find the angle ACB.
No pictures are given



3. The attempt at a solution

I have attempted the question by naming the triangle ABC with each angle opposite the line with the same name.
The I have changed A B and C into vector equations and used the scalar product between A and B to get the angle C.
This does not seem to be working.
I end up with

-8=√56√19cos(C)

c=104.197


This seems like a bit of a crazy answer am I doing it wrong?
Phys.Org News Partner Science news on Phys.org
Mysterious source of ozone-depleting chemical baffles NASA
Water leads to chemical that gunks up biofuels production
How lizards regenerate their tails: Researchers discover genetic 'recipe'
HallsofIvy
#2
Apr10-12, 07:39 AM
Math
Emeritus
Sci Advisor
Thanks
PF Gold
P: 39,491
Quote Quote by mazz1801 View Post
1. The problem statement, all variables and given/known data

Triangle ABC has A (−1, 3,−3), B (2, 4, 6) and C (3, 0,−5). Use the scalar
product to find the angle ACB.

No pictures are given



3. The attempt at a solution

I have attempted the question by naming the triangle ABC with each angle opposite the line with the same name.
The I have changed A B and C into vector equations and used the scalar product between A and B to get the angle C.
This does not seem to be working.
I end up with

-8=√56√19cos(C)

c=104.197


This seems like a bit of a crazy answer am I doing it wrong?
I don't see how anyone can tell you whether what you did was wrong when you haven't said what you did! What did you get for the vectors? What did you get for the lengths of those vectors?
mazz1801
#3
Apr10-12, 08:04 AM
P: 23
sorry I thought I had given enough info.

a= -i+3j-3k
b= 2i+4j+6k
c= 3i -5k

to find andle ACB by scalar product
a.b=|a||b|cos(c)

a.b= -8
|a|= √56
|b|= √19

so
-8=√56√19cos(c)

cos(c)=(-8) / (√56√19)

so c=cos-1((-8) / (√56√19))
c=104.197


Register to reply

Related Discussions
Vectors triangle Introductory Physics Homework 3
The height of a triangle, given 2 vectors and the area Calculus & Beyond Homework 2
Vectors and medians of a triangle. Calculus & Beyond Homework 1
Interactive file naming for naming multiple files in fortran90 Programming & Computer Science 2
Area of a triangle formed by vectors Precalculus Mathematics Homework 5