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Calculus II - Trigonometric Substitutions

  1. Aug 2, 2011 #1
    1. The problem statement, all variables and given/known data

    Evaluate

    integral dx/sqrt(x^2-49)

    2. Relevant equations



    3. The attempt at a solution

    SEE POST #4 BELOW WITH ATTACHMENT TO VIEW MY SOLUTION EASIER TO READ

    i was trying to use a csc(theta) substitution and I don't see how my logic and math below is flawed but apparently I'm getting a different answer, note that I wanted to try and evaluate this integral this way and not other ways and don't see how evaluating it this way is wrong as long as my intermediate steps are mathematically correct

    integral dx/sqrt(x^2-49)
    sense csc(theta)= hypotenuse/adjacent
    let hypotenuse = x
    and adjacent = 7
    use the fact that hypotenuse= adjacent^2 + opposite^2
    then opposite = sqrt(x^2-49)
    then csc(theta)=x/7
    then x = 7csc(theta)
    use the fact that d/dx csc(theta) = -cot(theta)csc(theta)
    dx/dtheta = -7cot(theta)csc(theta)
    dx=-7cot(theta)csc(theta)dtheta
    substitute dx and x back into the integral
    integral[ (-7cot(theta)csc(theta)dtheta)/sqrt((7csc(theta))^2-49) ]
    took out the constants and squared the term
    -7*integral[ (cot(theta)csc(theta)dtheta)/sqrt(49csc(theta)^2-49) ]
    factored out 49
    -7*integral[ (cot(theta)csc(theta)dtheta)/sqrt(49(csc(theta)^2-1)) ]
    use fact that cot(theta)^2=csc(theta)^2-1
    -7*integral[ (cot(theta)csc(theta)dtheta)/sqrt(49cot(theta)^2) ]
    evaluated the square root
    -7*integral[ (cot(theta)csc(theta)dtheta)/(7cot(theta)) ]
    "canceled" cot(theta)/cot(theta)=1 and took out constant and got 7/7=1
    - integral csc(theta)dtheta
    used the fact that integral csc(theta)dtheta = -ln(cot(theta)+csc(theta))
    -*-ln(cot(theta)+csc(theta)) + c
    it became positive
    ln(cot(theta)+csc(theta)) + c
    already established that csc(theta)= x/7
    ln(cot(theta)+x/7) + c
    use fact that cot(theta)=adjacent/opposite = 7/sqrt(x^2-49)
    ln( 7/sqrt(x^2-49) + x/7 ) + c

    so this gives me the wrong answer the correct one is
    ln(sqrt(x^2-49)/7 + x/7 ) + c
    but i really don't see how what i did is wrong...

    SEE POST #4 BELOW WITH ATTACHMENT TO VIEW MY SOLUTION EASIER TO READ
     

    Attached Files:

    Last edited: Aug 2, 2011
  2. jcsd
  3. Aug 2, 2011 #2
    Okay, this may not be it, but it's hard enough to follow trig substitutions, let alone follow it typed out instead of natural images, but....
    Why are you making a CDC substation?
    Thank you iPad..... I had to leave that autocorrect.
    Why are you using a csc substitution? Shouldn't it be a sec sub? X should equal 7sec(@).
    Hey, maybe using @ for theta will catch on.
     
  4. Aug 2, 2011 #3
    i don't know i wanted to try using a csc subsitution
    i'll make a good image
     
  5. Aug 2, 2011 #4
    here
    i guess just see my reasoning above if you don't understand what i did
    SEE ATTACHMENT IN THIS POST TO SEE A EASIER TO READ SOLUTION
    note that in the last expression i switched around co secant and cotangent as far as there order in there previous expression but everything is correct with the last expression I believe
     

    Attached Files:

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    Last edited: Aug 2, 2011
  6. Aug 2, 2011 #5
    Gotta be a rebel, huh? Alright I'll look at it again. Let me get out an actual piece of paper here...

    That was so much easier to follow! *still looking
     
  7. Aug 2, 2011 #6
    Alright thank you ^_^ I hope you can read my chicken scratch in attachment #4 lolz, let me know if you can't
     
  8. Aug 2, 2011 #7
    X=7csc@
    X/7=csc@
    Csc@=1/sin@ so
    Sin@=7/x
    Sin@=opp/hyp
    So 7 is the opposite , your triangle pic is backwards.
     
  9. Aug 2, 2011 #8
    Kinda had to figure it was something like that, since your answer was just kinda upside down.

    I hate trig subs. Avoid 'em like the plague.
     
  10. Aug 2, 2011 #9
    *smashes head into desk*
    lol thanks

    it's been to long sense i have done basic trig stuff
    and ya like i guess it's custom to use secant substitution in this case but i wanted to see if it really matter if i used co secant and I'm glad to see that it doesn't
     
  11. Aug 2, 2011 #10
    Hey, I learned you can use the *off brand* too. So it was worth my time. Thank you.
     
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