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Math Help - Just a simple order of operations question

  1. #16
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    Re: Just a simple order of operations question

    Quote Originally Posted by SplashDamage View Post
    In this example (which is actually from a previous test that i failed hard)
    ...
    I am not sure what to do.
    Is that your work there? Because that looks good to me (so long as we agree that x is positive and that y and z are nonzero).

    You are correct when you say that raising a product to an nth power is the same as multiplying that product by itself n times. The result of this is that the power gets "distributed" to each factor. That is, in general,

    (ab)^n=\overbrace{(ab)(ab)(ab)\cdots(ab)}^{n\ \mathrm{factors}}=a^nb^n.

    Another useful property is

    (a^m)^n = a^{m\cdot n} = a^{mn}.

    So,

    \frac{27z^9\left(y^2x^{-1/2}\right)^2}{\left(3z^3x^{-1/3}\right)^3y^4}

    =\frac{27z^9\left(y^2\right)^2\left(x^{-1/2}\right)^2} {\left(3\right)^3\left(z^3\right)^3\left(x^{-1/3}\right)^3y^4}

    =\frac{27z^9y^4x^{-1}}{27z^9x^{-1}y^4} = 1,

    again with x>0 and y,z\neq0.


    Edit: But be sure you see that we cannot distribute powers over addition or subtraction. That is, (a+b)^n\neq a^n+b^n for arbitrary a and b (though there are specific values for which this will be true). In this case, we would have to multiply (a+b) by itself n times.
    Last edited by Reckoner; May 30th 2012 at 08:01 AM.
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    Re: Just a simple order of operations question

    Quote Originally Posted by SplashDamage View Post
    I mean, if you had something like (x + 3) (y + 4) (x + y)... How do you multiply them all together? Like you would with just two.
    Multiply any two factors together. This will give you a new expression with one fewer factor than the previous one. Then repeat the process until everything is multiplied out and you are left with only one factor.

    To use your example,

    (x+3)(y+4)(x+y)

    =\left[(x+3)(y+4)\right](x+y)

    =(xy+4x+3y+12)(x+y)

    Now we distribute,

    =xy(x+y)+4x(x+y)+3y(x+y)+12(x+y)

    =x^2y + xy^2 + 4x^2 + 4xy + 3xy + 3y^2 + 12x + 12y

    and we could combine like terms. Using this procedure, you can multiply any finite number of polynomials together.
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    Re: Just a simple order of operations question

    Wow that is amazingly helpful. You're epic. What would happen if I ended up doing the ( ) ( ) ( ) thing depending on ^n? And would you recommend the way you did it over that method?

    Also another thing. This concerns getting the lowest common denominator in order to -/+ fractions together.

    For example I am confronted with this. (1/2) - (2/x+4)

    The way I was taught to find the LCD was split the lower halves into their prime factors, which I thought was 2 and 2^2. Although I think I got that wrong. Can you explain to me how to properly find the LCD as my lecturer clearly fails at explaining these things.
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    Re: Just a simple order of operations question

    Quote Originally Posted by SplashDamage View Post
    Wow that is amazingly helpful. You're epic. What would happen if I ended up doing the ( ) ( ) ( ) thing depending on ^n? And would you recommend the way you did it over that method?
    Yes, if you have an expression with more than one term, you don't have much choice other than multiplying the expression by itself over and over again.

    (a+b)^n = \overbrace{(a+b)(a+b)(a+b)\cdots(a+b)}^{n\ \mathrm factors},

    and then multiply everything out.

    Quote Originally Posted by SplashDamage View Post
    For example I am confronted with this. (1/2) - (2/x+4)

    The way I was taught to find the LCD was split the lower halves into their prime factors, which I thought was 2 and 2^2. Although I think I got that wrong. Can you explain to me how to properly find the LCD as my lecturer clearly fails at explaining these things.
    A simple procedure for finding the LCD is to completely factor each denominator (so that you have prime factors), and then take the highest power of each factor from either denominator. For example, to find the LCD of

    \frac1{3a^2+6ab+3b^2},\quad\frac1{4a^2+4ab},\quad \mathrm{and}\quad\frac1{2a^3}

    Factor each denominator completely:

    3a^2+6ab+3b^2 = 3\left(a^2+2ab+b^2\right) = 3(a+b)^2

    4a^2+4ab=4a(a+b)=2^2a(a+b)

    2a^3 = 2a^3

    To find the LCD, multiply the highest power of each factor:

    \mathrm{LCD} = 2^2\cdot3^1\cdot a^3\cdot(a+b)^2 = 12a^3(a+b)^2.

    For your example, I assume you mean \frac12 - \frac2{x+4} and not \frac12 - \left(\frac2x + 4\right)?

    In this case the denominators are already factored completely. And since there are no common factors, our LCD would simply be the product: \mathrm{LCD}=2(x+4).
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    Re: Just a simple order of operations question

    Quote Originally Posted by Reckoner View Post

    To find the LCD, multiply the highest power of each factor:

    \mathrm{LCD} = 2^2\cdot3^1\cdot a^3\cdot(a+b)^2 = 12a^3(a+b)^2.
    Not sure what you did/meant here

    and yeah I meant 2 / (x + 4)
    Last edited by SplashDamage; May 30th 2012 at 08:33 AM.
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    Re: Just a simple order of operations question

    Also I figured at if we had 2 and (x + 4)...

    wouldnt the factors be 2, x, 4

    and then wouldnt it be 2 * x * 4, making 8x?
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    Re: Just a simple order of operations question

    Quote Originally Posted by SplashDamage View Post
    Not sure what you did/meant here
    The first denominator is: 3\cdot(a+b)^2

    The second: 2^2\cdot a\cdot(a+b)

    And the third: 2\cdot a^3

    In all of these we have four different factors: 2, 3, a, and (a+b).

    So we first look for the highest power of 2. The first denominator has zero factors of 2 ( 2^0), the second one has two factors ( 2^2), and the third denominator has one factor ( 2^1). In our LCD, we use the highest power of 2 among these three, so we take 2^2. Similarly, 3^1 is the highest power of 3 (the other denominators have zero factors of three), a^3 is the highest power of a, and (a+b)^2 is the highest power of (a+b). We take each of these and multiply them together.
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    Re: Just a simple order of operations question

    OMG i get it. That makes the other question I just made clear.
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    Re: Just a simple order of operations question

    Quote Originally Posted by SplashDamage View Post
    Also I figured at if we had 2 and (x + 4)...

    wouldnt the factors be 2, x, 4
    No, factors are the things that are being multiplied to form the final product. x and 4 are being added, not multiplied. So (x+4), in its entirety, is a single factor, and cannot be split further.
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    Re: Just a simple order of operations question

    So something like 5x can be split into 5 and x, or something like 25x could be split into 5^2 and x. And two factors can be grouped as one factor if there is parenthesis around them for example (x + 4)?

    and if that parenthesis was not there, would the LCD indeed be 4x?

    actually no it would still be 2 (x + 4) wouldnt it.
    Last edited by SplashDamage; May 30th 2012 at 09:16 AM.
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    Re: Just a simple order of operations question

    Quote Originally Posted by SplashDamage View Post
    So something like 5x can be split into 5 and x, or something like 25x could be split into 5^2 and x.
    Yes
    Quote Originally Posted by SplashDamage View Post
    And two factors can be grouped as one factor if there is parenthesis around them for example (x + 4)?
    I'm not sure I understand what you mean. We can group factors as we please, for multiplication is associative: abc=(ab)c=a(bc). x+4 is a single factor, however (though it consists of two terms).
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    Re: Just a simple order of operations question

    So with or without the brackets, it would still be 2(x + 4)?
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    Re: Just a simple order of operations question

    Quote Originally Posted by SplashDamage View Post
    So with or without the brackets, it would still be 2(x + 4)?
    Right. When x+4 is by itself, we don't need to write the parentheses. But it's still a single factor, and we can't factor it further.
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    Re: Just a simple order of operations question

    What would happen if we had (x + 8) instead? since 8 can be factored.

    also if we had x + 8
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    Re: Just a simple order of operations question

    Quote Originally Posted by SplashDamage View Post
    What would happen if we had (x + 8) instead? since 8 can be factored.
    8 can be factored as 2^3, yes. But even though 2 is a factor of 8, it is not a factor of (x+8) (because it is not a factor of x). Since x and 8 have no common factors, the overall expression (x+8) cannot be factored any further.

    If, on the other hand, we had something like (2x+8), we could factor out a 2 from each term:

    2x+8 = 2\cdot x+2\cdot4 = 2(x+4).
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