# vectors and sclalar products

• Jan 16th 2010, 03:12 AM
Oasis1993
vectors and sclalar products
The points A,B and C have position vectors given respectively
a = ( 7i + 4j -2k) b= ( 5i + 3j - 3k) c= ( 6i + 5j - 4k)

a) Find the angle BAC
b) Find the area of the triangle ABC

i have dont part a) and got 60 degrees but i cant do part b)

• Jan 16th 2010, 03:21 AM
pickslides
This site has a good method

Area of a Triangle via the Cross Product
• Jan 16th 2010, 07:04 AM
Soroban
Hello, Oasis1993!

Quote:

The points A,B and C have position vectors given respectively:
. . $\displaystyle a = \langle7,4,-2\rangle,\;\;b = \langle5,3,-3\rangle, \;c = \langle6, 5,-4\rangle$

b) Find the area of $\displaystyle \Delta ABC.$

You are expected to know this fact:

The area of the parallelogram determined by two vectors
. . is the magnitude of the cross product of the two vectors.
Code:

          * . . . . . .         /          .       v /          .       /          .       * - - - - - *             u   Area  =  |u x v|

Hence, the area of the triangle determined by two vectors is: .$\displaystyle \tfrac{1}{2}|\vec u \times \vec v|$

Let: .$\displaystyle \begin{array}{ccccc}\vec u &=& \overrightarrow{ab} &=& \langle \text{-}2,\text{-}1,-1\rangle \\ \vec v &=& \overrightarrow{bc} &=& \langle 1,2,\text{-}1\rangle \end{array}$

Then: .$\displaystyle \vec u \times \vec v \;-\;\begin{vmatrix}i&j&k \\ \text{-}2&\text{-}1&\text{-}1 \\ 1&2&\text{-}1 \end{vmatrix} \;=\;3i - 3j - 3k \;=\;\langle3,\text{-}3,\text{-}3\rangle$

$\displaystyle \text{Area of parallelogram}\;=\;\sqrt{3^2 + (\text{-}3)^2+(\text{-}3)^2} \;=\;\sqrt{27} \;=\;3\sqrt{3}$

. . $\displaystyle \text{Therefore: area of triangle} \:=\:\frac{3\sqrt{3}}{2}$

• Jan 17th 2010, 02:08 AM
Oasis1993
Hello thanks for your answer but i dont understand what you did here:
Then: .http://www.mathhelpforum.com/math-he...e6ecc339-1.gif