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Dodo1

  • 2 years ago

Please help me. Domain of sqrt ((x-8)(x+4)) thanks

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  1. mikoza
    • 2 years ago
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    is (-infinity ,-4]U[8,+infinity)

  2. Dodo1
    • 2 years ago
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    i entered it and it told me that it was wrong

  3. davisla
    • 2 years ago
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    Answers without solutions don't help understanding so then you can't do the next one or the one on your exam. The value we are taking the square root of must be greater than or equal to 0, so in the expression \[\sqrt{(x-8)(x+4}\] the radicand is (x-8)(x+4) set up the inequality \[(x-8)(x+4)\ge0\]

  4. davisla
    • 2 years ago
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    Now, the only way we can go from positive to negative is to pass through 0 and the only way we can go from negative to positive is to pass through 0, so find the zeros. For what values of x is (x - 4)(x+8) = 0?

  5. mikoza
    • 2 years ago
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    dear davisla (x-4)(x+8)is not equal to (x-8)(x+4)

  6. davisla
    • 2 years ago
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    you are correct ... so for what values of x is (x - 8)(x + 4) = 0?

  7. Dodo1
    • 2 years ago
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    I thought that X=-8. and X=+4. I am not sure how the infinity blacket works

  8. Dodo1
    • 2 years ago
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    sorry I mean X=+8 and X=-4

  9. davisla
    • 2 years ago
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    once you find the zeros, you know where it COULD change signs. That does not mean it does. So, from neg infinity to -4 it NEVER changes signs, from -4 to 8 it NEVER changes signs and from 8 to infinity it NEVER changes signs Plug in a number in each of those 3 intervals to determine the sign of the expression in that interval. Try -5: (-5 - 4)(-5 + 4) = -9 (-1) which is +9 ... but all we care about is that it is positive. So in the interval (-infinity, -4) it is positive - that is okay so that is part of the domain. Now try a number in the second interval and see if it is positive or negative.

  10. Dodo1
    • 2 years ago
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    so the second interval should be (8,infinity)?

  11. davisla
    • 2 years ago
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    the second is (-4, 8) the third is (8, infinity)

  12. davisla
    • 2 years ago
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    in the second interval, I would choose 0 to plug in since it is easy

  13. Dodo1
    • 2 years ago
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    Ok so, the answer should be (-infinity,-4)U(-4,0)U(0,8)U(8,infinity)?

  14. davisla
    • 2 years ago
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    no, when you plug 0 in you get: (0-8)(0+4) = -8(4) and that is a negative number - so that interval is no good and is not in your domain what about the last interval? do you get a positive or a negative?

  15. Dodo1
    • 2 years ago
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    I got positive.

  16. davisla
    • 2 years ago
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    so that interval is in our domain ... now, what about -4 and 8? Can the expression = 0?

  17. davisla
    • 2 years ago
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    we can take the sq root of zero, so we do want to include those endpoints in our intervals

  18. davisla
    • 2 years ago
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    we wind up with the intervals given by someone else in the beginning and that is correct. \[(-\infty,-4]\cup[8,\infty)\]

  19. davisla
    • 2 years ago
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    I am wondering if it didn't like the way you input ... did you put a + in front of the second infinity?

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