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  1. #1
    Registered Users Regular Rayne's Avatar
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    i feel pretty good about my calculus III final. only i know i got one question slightly wrong. to integrate a function we had to find an inverse function, i KNOW that i got it wrong but i still cant understand WHY i got it wrong.

    the function was the square root of x/3. i had the inverse function as 9x^2. the right answer is 3x^2 but i cant understand why. in the original function, you take x, divide it by three and take the root of EVERYTHING. its inverse should: take x, multiply it by 3 THEN square everything. 9x^2. but i can compute some examples and show that 3x^2 IS the right inverse, but cant figure out where my reasoning is wrong.
    you multiply it by the root of three and then square everything, not three.

  2. #2
    what about .. eyebrows God's Avatar
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    well, yeah, obviously that is a way to get the right answer, i still dont fully understand where my reasoning was wrong. did i do the 'order of operations' if you will wrong? like, do you not only have to do inverse operations but also in the reverse order.

    for example: original function: take x, divide it by three, take square root of everything. therefore, inverse function: take x, square everything, multiply by three, in that order. this is probably it.

  3. #3
    Annnnzyle and Itterpretnl Seven's Avatar
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    I always just go from y=sqrt(x/3) and then swap x and y and then solve for the new y.
    777

  4. #4
    what about .. eyebrows God's Avatar
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    yes, there are many ways to get the right answer, none of which i did because i was somewhat rushed and just did what seemed logical. i know that it was wrong, and i know things to do that would have gotten me the right answer, but just was questioning what was wrong in my particular line of reasoning.

  5. #5
    what about .. eyebrows God's Avatar
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    out of curiosity rayne, how far have you gotten in math topics so far

  6. #6
    Annnnzyle and Itterpretnl Seven's Avatar
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    Quote Originally Posted by God View Post
    yes, there are many ways to get the right answer, none of which i did because i was somewhat rushed and just did what seemed logical. i know that it was wrong, and i know things to do that would have gotten me the right answer, but just was questioning what was wrong in my particular line of reasoning.
    Oh, then I'd need a pen and paper, but I don't think I have any so I'll leave Rayne to answer.
    777

  7. #7
    Registered Users Regular Rayne's Avatar
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    Quote Originally Posted by God View Post
    well, yeah, obviously that is a way to get the right answer, i still dont fully understand where my reasoning was wrong. did i do the 'order of operations' if you will wrong? like, do you not only have to do inverse operations but also in the reverse order.

    for example: original function: take x, divide it by three, take square root of everything. therefore, inverse function: take x, square everything, multiply by three, in that order. this is probably it.
    yes you have to do it in the reverse order

  8. #8
    Registered Users Regular Rayne's Avatar
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    a better way to think about this problem is instead of taking it in terms of √(x/3) imagine it to be 1/√3 times √x - ie, take the root of x and then divide it by root three. to get back the original function do the step in reverse: multiply it by √3 and then square everything.

    so the inverse function is (x√3)^2 which is 3x^2.

    you should generally avoid making taking the root of constants a part of the function. just imagine the roots as one whole constant. it prevents shit from hitting the fan; roots are pretty finicky.

  9. #9
    Registered Users Regular Rayne's Avatar
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    Quote Originally Posted by God View Post
    out of curiosity rayne, how far have you gotten in math topics so far
    basically finished around 80% of whats there in khanacademy's maths stuff the exceptions being bits of calculus and linear algebra and a lot of advanced statistics.

    side note: i might fail my maths exam.

  10. #10
    what about .. eyebrows God's Avatar
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    what are your math exams like? i dont suppose you have any sort of practice exam sort of thing file online?

  11. #11
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    When in doubt reverse the order.

    Flame,Troll and Spoil

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    Annnnzyle and Itterpretnl Seven's Avatar
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    A stitch in time saves nine.
    777

  13. #13
    Registered Users Regular Rayne's Avatar
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    Quote Originally Posted by God View Post
    what are your math exams like? i dont suppose you have any sort of practice exam sort of thing file online?
    http://cbse.nic.in/curric~1/sqpms-ma...2010-part1.pdf

    two sample papers and solutions

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    Registered Users Regular Rayne's Avatar
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    princeso Kirby's Avatar
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    Quote Originally Posted by Rayne View Post
    I have to use this.

  16. #16
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    I lost 3 minutes that I'll never get back reading that.

    Flame,Troll and Spoil

  17. #17
    Annnnzyle and Itterpretnl Seven's Avatar
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    Quote Originally Posted by Lord Trigon View Post
    I lost 3 minutes that I'll never get back reading that.
    Typically time spent is always time lost, seeing as you can't regain it.
    777

  18. #18
    Geekwad RangerGirl's Avatar
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    The concept of taking Calc III makes my soul bleed.
    "What luck for rulers that men do not think." ~Adolf Hitler
    "you are gay furthermore you like sex with men" ~god

    One of the Seven Deadly Sins of PD: Femme Greed

  19. #19
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    Quote Originally Posted by Seven View Post
    Typically time spent is always time lost, seeing as you can't regain it.
    true but you know what is a good read, looking up poisonous plants. I found out I had collected nightshade when I was a kid.

    Flame,Troll and Spoil

  20. #20
    Registered Users Regular Rayne's Avatar
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    Quote Originally Posted by Rayne View Post
    btw did you actually try doing these? i think youve covered most of the shit in it.

  21. #21
    Registered Users Regular Rayne's Avatar
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    ??

    a YES or NO would suffice

  22. #22
    what about .. eyebrows God's Avatar
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    not really. i should though.

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