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Math Help - THIS is where I am confused about triangle vector calculation

  1. #1
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    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.

    I`m going to take this example out of my book because it doesn`t 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?
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  2. #2
    Super Member Deadstar's Avatar
    Joined
    Oct 2007
    Posts
    722
    Quote Originally Posted by thekrown View Post
    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.

    I`m going to take this example out of my book because it doesn`t 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?
    \sqrt{100+25+100} = \sqrt{225} = 15...
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