
Algebra PreAlgebra and Basic Algebra Math Forum 
 LinkBack  Thread Tools  Display Modes 
November 5th, 2014, 10:14 AM  #1 
Newbie Joined: Oct 2014 From: Marilia Posts: 6 Thanks: 0  find area of triangle (linear algebra )
Guys, good afternoon! I'm racking my brain with this: a) Obtain the area of the triangle vertices A ( 1,0,1 ) B ( 0,2,3 ) and C ( 2,0,1 ) b ) Use the result of the area to FIND the height of the vertex C to the side AB. Can anyone enlighten me with making the resolution of this exercise? Last edited by skipjack; November 5th, 2014 at 06:05 PM. 
November 5th, 2014, 10:24 AM  #2 
Newbie Joined: Nov 2014 From: mississauga Posts: 2 Thanks: 0 
Could you create 2 vectors with those 3 coordinates, then use 1/2 the magnitude of the cross product? Last edited by skipjack; November 5th, 2014 at 06:05 PM. 
November 5th, 2014, 12:38 PM  #3  
Senior Member Joined: Jul 2014 From: भारत Posts: 1,178 Thanks: 230 
Imitating $\displaystyle \color{green}{\textbf{soroban}}$'s way of posting. Hello, Juan Victor! Quote:
The magnitude of the cross product of two vectors is the area of the parallelogram which has adjacent sides equal to the magnitudes of the two vectors. We form the vectors $\displaystyle \overline{\text{AB}}$ and $\displaystyle \overline{\text{AC}}$ $\displaystyle \overline{\text{AB}}= \bigg( \ 01 \ , \ 20 \ , 31 \bigg)= \bigg( \ 1 \ , \ 2 \ , \ 2 \ \bigg)$ $\displaystyle \overline{\text{AC}}= \bigg( \ 21 \ , \ 00 \ , 11 \bigg)= \bigg( \ 1 \ , \ 0 \ , \ 0 \ \bigg)$ We find their cross product, i.e., $\displaystyle \overline{\text{AB}} \times \overline{\text{AC}}$ $\displaystyle \begin{array}{ r c c } \ i & j & k \ \\ 1 & 2 & 2 \ \\ \ 1 & 0 & 0 \ \end{array}= \Big[ \ (2)(0)(2)(0) \ \Big] \overline{\text{i}} \  \ \Big[ \ (1)(0)(2)(1) \ \Big] \overline{\text{j}} \ + \ \Big[ (1)(0)(2)(1) \ \Big] \overline{\text{k}}$ $\displaystyle = 0 \overline{\text{i}} \  \ (02) \overline{\text{j}} \ + \ (12) \overline{\text{k}}$ $\displaystyle = \bigg( \ 0 \ , \ 2 \ , 3 \ \bigg)$ The magnitude of $\displaystyle = \bigg( \ 0 \ , \ 2 \ , 3 \ \bigg)$ is $\displaystyle \sqrt{0^{2}+2^{2}+(3)^{2}}= \sqrt{0+4+9}= \sqrt{13}$ The triangle's area is $\displaystyle \frac{1}{2} $ the area of the parallelogram. Can you proceed? Can you solve the second part of the question? Last edited by skipjack; November 5th, 2014 at 06:12 PM.  
November 5th, 2014, 06:09 PM  #4 
Global Moderator Joined: Dec 2006 Posts: 19,722 Thanks: 1807 
The method used above is okay, but there's a slip in the working. Alternatively, use the easily found "height" from vertex B to side AC and the fact that AC has length 1. 
November 6th, 2014, 01:15 AM  #5 
Newbie Joined: Oct 2014 From: Marilia Posts: 6 Thanks: 0 
Yes, Thanks Prakhar!

November 6th, 2014, 01:32 AM  #6 
Newbie Joined: Oct 2014 From: Marilia Posts: 6 Thanks: 0 
Prakrar only one observation, [(1 )(0 )  (2 )(1)] k = 0i  (02)j + ( 12 )k . = ( 0, 2, 3) The value of the last element of the vector k is related to the 2 and 3 do not agree? 
November 6th, 2014, 06:10 AM  #7 
Global Moderator Joined: Dec 2006 Posts: 19,722 Thanks: 1807 
You've found prakhar's slip... the last line should be = (0, 2, 2).


Tags 
algebra, area, find, linear, triangle 
Search tags for this page 
Click on a term to search for related topics.

Thread Tools  
Display Modes  

Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Area of triangle ABC  Albert.Teng  Algebra  6  February 24th, 2013 09:08 PM 
Area of triangle AEF  Albert.Teng  Algebra  5  February 19th, 2013 10:52 PM 
Right triangle area  Denis  Algebra  5  October 4th, 2012 03:40 AM 
Find the area of ??triangle  zgonda  Algebra  4  September 26th, 2011 10:27 AM 
area of eq. triangle vs. area of square  captainglyde  Algebra  1  February 19th, 2008 08:55 AM 