# Thread: THIS is where I am confused about triangle vector calculation

1. ## THIS is where I am confused about triangle vector calculation

Okay so I have a thread posted about my confusion regarding calculating the area of a triangle.

Long story short, I was confused what to do with the cross product to get the area.

Im going to take this example out of my book because it doesnt make any sense to me.

We have three points:

P (2,2,0)
Q (-1,0,2)
R (0,4,3)

PQ (-3,-2,2)
PR (-2,2,3)

PR x PR (-10,5,-10)

I can do this myself, and this is what the book gives as answers aswell.

What really gets me going is I know, both from my teacher in class and this forum that I simply put the cross product into a formula and we get this:

(1/2) | square root of a^2 + b^2 + c^2 | (magnitude)

which is in this case (1/2) | square root of 100 + 25 + 100 |

I've always been solving this type of question like this, however... the book says the anser is 15/2. How did they get this? How come they did not find the magnitude and then divide it by half?

2. Originally Posted by thekrown
Okay so I have a thread posted about my confusion regarding calculating the area of a triangle.

Long story short, I was confused what to do with the cross product to get the area.

Im going to take this example out of my book because it doesnt make any sense to me.

We have three points:

P (2,2,0)
Q (-1,0,2)
R (0,4,3)

PQ (-3,-2,2)
PR (-2,2,3)

PR x PR (-10,5,-10)

I can do this myself, and this is what the book gives as answers aswell.

What really gets me going is I know, both from my teacher in class and this forum that I simply put the cross product into a formula and we get this:

(1/2) | square root of a^2 + b^2 + c^2 | (magnitude)

which is in this case (1/2) | square root of 100 + 25 + 100 |

I've always been solving this type of question like this, however... the book says the anser is 15/2. How did they get this? How come they did not find the magnitude and then divide it by half?
$\displaystyle \sqrt{100+25+100} = \sqrt{225} = 15$...