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Thread: Algebra help.

  1. #1

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    Algebra help.

    Can any of you check this out for me. i started getting back into math again and i kind of suck at it. This is a question out of one of my text books and they come up with a diffrent answer and i think they are wrong.

    2x^3 - x^2 + y where x = -3 & y = 2

    I come up with:

    2(-3)^ 3 - (-3)^2 + 2

    2(-27) - (-9) + 2

    (-54) - (-9) + 2 = 61

    they come up with

    2(-3)^ 3 - (-3)^2 + 2

    2(-27) - (9) + 2

    -54 - 9 + 2 = -61

    but wouldn't (-3^2) be (-9) not 9?


    Thanks

  2. #2
    PowerPoster beachbum's Avatar
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    hi
    A negative number to the power of an even number returns a positive. So, -3^2 = 9, -3^3 = -27 and -3^4=81


    2x^3 - x^2 + y where x = -3 & y = 2

    2(-3^3) - (-3^2) + 2
    2(-27) - (9) + 2
    -54 - 9 + 2
    = -61

    Regards
    Stuart
    Stuart Laidlaw
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  3. #3
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    Code:
    3 + (-3)  = 0
    
    so as anything * 0 = 0
    
    3 * (3 + -3) = 0 = (3 + -3) * -3
    
    
    multiplying out
    (3 * 3) + ( 3 * -3)  = ( 3 * -3) + ( -3 * -3) 
    
    
    and we can subtract ( 3 * -3) from both sides
    
    (3 * 3) = ( -3  * -3)
    
    (-3)^2  = 9
    If it wasn't for this sentence I wouldn't have a signature at all.

  4. #4

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    So the book was basically wrong. I didn't know that a negative number to the power of an even number returns a positive. Thanks that will come in handy.

  5. #5
    PowerPoster beachbum's Avatar
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    Originally posted by Dilenger4
    So the book was basically wrong. I didn't know that a negative number to the power of an even number returns a positive. Thanks that will come in handy.
    LOL no, the book was basically right.. you were basically wrong
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  6. #6
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    Beachbum don't state it like a formula that a negative number squared will give you a positive number. Explain him the logic.

    Let's do it with 1

    (+1) x (+1) = +1

    (+1) x (-1) = -1

    (-1) x (+1) = -1

    (-1) x (-1) = +1

    So you see that.. if you multiply a negative number 'a' to another number 'b' the resultant product 'c' has the opposite sign of 'b'.

    There fore 1 x -1 x 1
    can be processed as

    1 x -1 = -1 which in turned multiplied by the third number 1

    -1 x 1 = -1

    like this you can see (-1)2 = -1 x -1 = +1

    while (-1)3 = -1 x -1 x -1 = -1
    Marriage - is not a word, but a sentence.

  7. #7

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    LOL no, the book was basically right.. you were basically wrong
    How do you figure? (-3)^2 = -9 which i came up with. The book came up with 9.

  8. #8
    PowerPoster beachbum's Avatar
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    Originally posted by thinktank
    Beachbum don't state it like a formula that a negative number squared will give you a positive number. Explain him the logic.
    as Jamie would say ... pfffft!!

    Dilinger no that isnt true.. - 3 ^ 2 = -3 x -3 = 9
    Look at Thinktank's post for explanation... and then feel free to say Bollox to him as well!!
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  9. #9

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    thinktank yes you are right. So i guess i could think of it this way.

    When multiplying numbers with the same sign, multiply the absoulte value of each number. The product will always be positive whether the numbers are bolth positive or bolth negative

    The same for numbers with diffrent signs but the result will always be negative.

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    Marriage - is not a word, but a sentence.

  11. #11
    PowerPoster beachbum's Avatar
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    Sorry Dilinger but u are getting ur negatives mixed up

    -3 ^ 3 = -27
    2 x -27 = -54 so lets store that away

    -3 ^ 2 = 9 (-3 x -3)

    and finally +2

    so the formula is now -54 - (9) + 2 = -61
    The minus comes from the formula itself ie
    2x^3 - x^2 + y
    and not fromthe result of x^2

    Regards
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  12. #12
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    Originally posted by Dilenger4
    So i was right on the 2(-27) - (-9) + 2 part because (-3)^2 does in fact equal -9 but i was wrong in calculating (-54) - (-9) + 2 = which equals -61 not 61 like i came up with.

    But how do we calculate (-54) - (-9) + 2 = ?
    I guess you are seriously weak in the fundamentals
    (-3)2 = -3 x -3 = +9

    -54 - (-9) = -54 -1(-9) taking -1 inside the bracket
    -54 + (-1 x -9)

    -54 + 9 = -45

    __________________

    -54 -(-9) + 2 = -54 + (-1 x -9) + 2 = -54 + 9 + 2 = -54 + 11

    = -43
    Marriage - is not a word, but a sentence.

  13. #13

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    yes you are right (-54) - (-9) + 2 = -61 I guess the surgeon general was right smoking marijuana does burn brain cells.

  14. #14
    PowerPoster beachbum's Avatar
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    Originally posted by Dilenger4
    yes you are right (-54) - (-9) + 2 = -61 I guess the surgeon general was right smoking marijuana does burn brain cells.
    damn u Dilenger!!! arrghhh

    -54 - 9 + 2 = -61
    -54 - (-9) + 2 = -43

    Stuart Laidlaw
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  16. #16
    Banned plenderj's Avatar
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    Pffft you bunch of morons. Bollox to yiz!
    Christ you're arguing over algebra... its not like art lads. There's a right way, and a wrong way.

    *mutter*stuart*mutter*wrong way*mutter*

  17. #17
    PowerPoster beachbum's Avatar
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    No arguments here... just lots of help, compassion and brotherly love. I dont know much about algebra but i know what i like
    Stuart Laidlaw
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  18. #18
    Banned plenderj's Avatar
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    Originally posted by beachbum
    i like just lots of brotherly love
    jesus christ ya puff!

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    we don't have jelly donuts in aus...

    do you mean jam donuts?

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    well here in the UK we call them doughnuts!
    There are 10 types of people in the world - those that understand binary, and those that don't.

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