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abz_tech

integration question

  • one year ago
  • one year ago

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  1. abz_tech
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    \[\int\limits_{3/-2}^{1/-2} dx/4x^{2}+12x+13\]

    • one year ago
  2. TuringTest
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    complete the square in the denominator and then trig substitute

    • one year ago
  3. abz_tech
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    i tried that but.. got stuck at the +13

    • one year ago
  4. abz_tech
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    got \[(x+2)^{2}+8\]

    • one year ago
  5. abz_tech
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    8 is not a square and if u factorise (x+2)^2 u dont get 4x^2....

    • one year ago
  6. TuringTest
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    factor that four out of the denom first\[\int_{-\frac32}^{-\frac12}\frac{dx}{4x^2+12x+13}=\frac14\int_{-\frac32}^{-\frac12}\frac{dx}{x^2+3x+\frac{13}4}\]\[=\frac14\int_{-\frac32}^{-\frac12}\frac{dx}{x^2+3x+\frac94+1}\]

    • one year ago
  7. TuringTest
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    \[=\frac14\int_{-\frac32}^{-\frac12}{dx\over(x+\frac32)^2+1}\]

    • one year ago
  8. abz_tech
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    how did u get 9/4+1? =/

    • one year ago
  9. TuringTest
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    13/4=(9/4)+1 completing the square on x^2+3x means adding a term of 9/4 you can save some trouble by noticing that 13/4=1+9/4 so we can make our constant term from the fraction we already have

    • one year ago
  10. abz_tech
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    can u continue plz

    • one year ago
  11. TuringTest
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    trig sub:\[x+\frac32=\tan\theta\implies dx=\sec^2\theta d\theta\]change bounds...

    • one year ago
  12. abz_tech
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    =/

    • one year ago
  13. dpaInc
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    |dw:1338250956092:dw|

    • one year ago
  14. abz_tech
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    to be honest i just want to know how my teacher got dx/4x^2+12x+13 to be dx/(2x+3)^2+2^2

    • one year ago
  15. abz_tech
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    \[4x ^{2}+12x+13 \to (2x+3)^{2}+2^2\] can u tell me how he got it =/..thx

    • one year ago
  16. TuringTest
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    complete the square\[4x^2+12x+13=4x^2+12x+9+4=(2x+3)^2+2^2\]knowing that splitting the terms up like this is not as obvious as the way in which I did it I think, so it requires a little more intuition I prefer to get the coefficient of the x term to be 1 to be safe, but notice that you can take my answer and your teachers result follows if you let the 4 back in the denom

    • one year ago
  17. TuringTest
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    he just noticed that splitting the 13=9+4 would work

    • one year ago
  18. TuringTest
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    above I derived it my way\[=\frac14\int_{-\frac32}^{-\frac12}\frac{dx}{x^2+3x+\frac94+1}\]let back in the 4 now...\[=\int_{-\frac32}^{-\frac12}\frac{dx}{4x^2+12x+9+4}\]from whence follows your teacher's way note both will give the same answer obviously

    • one year ago
  19. abz_tech
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    alright thx.. but the way u did is not in my course =/ i dont even know what changing bounds means..yet lol

    • one year ago
  20. TuringTest
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    the bounds in terms of x are -3/2 and -1/2 we make the substitution \[2x+3=2\tan\theta\]plug in the bounds of x and we can get them in terms of theta\[2(-\frac32)+3=0=2\tan\theta\implies \theta=0\]\[2(-\frac12)+3=2=2\tan\theta\implies\theta=\tan^{-1}1=\frac{\sqrt2}2\]so the new bounds are\[\int_{0}^{\frac{\sqrt2}2}\text{(whatever)}dx\]

    • one year ago
  21. abz_tech
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    :o..never did that.. before :P..kinda confusing.. we jsut call what u call bounds..limits

    • one year ago
  22. TuringTest
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    whatever floats your boat I just don't like getting that term confused with limits like\[\lim_{x\to a}f(x)\]

    • one year ago
  23. TuringTest
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    call 'em Freddies for all I care though, makes no mathematical difference whatever is easiest for you to keep straight

    • one year ago
  24. abz_tech
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    lol.. ya i guess, thx again see u around

    • one year ago
  25. TuringTest
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    welcome, good luck :)

    • one year ago
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