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Math Help - Cubic Equation

  1. #1
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    Cubic Equation

    Solve the cubic equation 3x^3-x^2-8x-4=0. Hence solve the equation 3(y+1)^3-(y+1)^2=8y+12

    I have already solved the cubic equation however the 2nd part makes no sense to me.

    Answers to cubic equation is (x+1)(3x+2)(x-2)

    MY GUESS is that the solution to solving part 2 is changing the y in 8y+12 to (y+1) so as to substitute y+1 = x
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  2. #2
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    Quote Originally Posted by Punch View Post
    Solve the cubic equation 3x^3-x^2-8x-4=0. Hence solve the equation 3(y+1)^3-(y+1)^2=8y+12

    I have already solved the cubic equation however the 2nd part makes no sense to me.

    Answers to cubic equation is (x+1)(3x+2)(x-2)

    MY GUESS is that the solution to solving part 2 is changing the y in 8y+12 to (y+1) so as to substitute y+1 = x
    3(y+1)^3-(y+1)^2=8y+12 \Rightarrow 3(y+1)^3-(y+1)^2 - 8(y+1) + 8 - 12 = 0 ....

    Then y + 1 = x ....
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  3. #3
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    sorry but how did you get the y+1 for the 8 and an extra 8 popping out?

    can you show me a more detailed working as i am not very expert in maths
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  4. #4
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    Quote Originally Posted by Punch View Post
    sorry but how did you get the y+1 for the 8 and an extra 8 popping out?

    can you show me a more detailed working as i am not very expert in maths
    You're expected to know that 8(y + 1) = 8y + 8 and therefore 8y = 8(y + 1) - 8.

    And it should be clear why I wrote 8y as 8(y + 1) - 8.

    By the way, I see from the time stamps that you spent exactly 2 minutes thinking about my reply (even less if the time taken to type your reply is taken into account). You're expected to put in much more effort than 2 minutes when trying to understand the help you get given.
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  5. #5
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    Quote Originally Posted by mr fantastic View Post
    By the way, I see from the time stamps that you spent exactly 2 minutes thinking about my reply (even less if the time taken to type your reply is taken into account). You're expected to put in much more effort than 2 minutes when trying to understand the help you get given.
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