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Homework Help: Arcsin(4/5)=x exact value

  1. Aug 11, 2010 #1
    1. The problem statement, all variables and given/known data

    I got down to here...

    x = i ln( 5/(4i + 3) ) + 2 pi n

    n set of integers positive and negative

    ok I was wondering if I could simplify this some more... i.e. I checked my answer in my calculator it was right just wondering that maybe...

    x = i ln( i^((-i 2 ln 5)/pi) / ( i^((-i 2 ln 4)/pi + 1) + i ^((-i 2 ln 3)/pi) ) )+ 2 pi n

    my gut tells me that it can be simplified some way or another if you turn all of the real numbers into complex numbers like with all numbers with the same base... but I can't seem to simplify... it's really bothering me as my gut tells me that there is a way as any fraction for the most part that have the same base with different exponents can be simplified...

    like if the equation was

    5^6/(5^2 + 5^8)

    could be simplifed so... I feel dumb I can't simplify this one further =(
    2. Relevant equations

    3. The attempt at a solution
    Last edited: Aug 11, 2010
  2. jcsd
  3. Aug 11, 2010 #2
    Just in case you guys are working on the same summer assignment... lol this is Dylan H. by the way... just incase teacher googles my work :O... it's been done before =(... no teacher that was me lol
  4. Aug 11, 2010 #3
    ok well lets see
    A^B + A^C = A^D(A^(B-D) + A^(C-D)
    hmmm... take out D from both... cancel it out... but what is D... :O
  5. Aug 12, 2010 #4
    Can someone help me siplify it further
  6. Aug 12, 2010 #5
    x = i ln( i^((-i 2 ln 5)/pi) / ( i^((-i 2 ln 4)/pi + 1) + i ^((-i 2 ln 3)/pi) ) )+ 2 pi n

    can i sipmlify the part in the natural log further with all of the exponents witin the natural log my gut tells me i can becasue they are now all in the same base... just can't figure out how to
  7. Aug 12, 2010 #6
    does somebody know how to simplify this further or if it's even possible?
  8. Aug 12, 2010 #7


    Staff: Mentor

    What's the question? Is it to find the exact value of x = arcsin(4/5)? What did you do to rewrite the equation as x = i ln( 5/(4i + 3) ) + 2 pi n? How is that simpler than arcsin(4/5)?
  9. Aug 12, 2010 #8
    it's the exact value of the solution
    I solved for x using eluer's formulas for sin and what not put it in your calculator and put in arcsin(4/5) it's right

    I rewrote it all in terms of i and was wondering how do I simplify
  10. Aug 12, 2010 #9
    it's not that it's simpler it's just the exact value instead of using
    .927295218 radians as it's wrong and only a appoximation one can find the exact value as I have done I then rewrote it all of it using complex numbers and now that everything has the same base I was wondering if somebody could help me simplify
  11. Aug 12, 2010 #10


    Staff: Mentor

    Why not leave it at arcsin(4/5)? I don't see any advantage in writing it in terms of logs and imaginary numbers.
  12. Aug 12, 2010 #11
    I was told to "evalutate" nomrally that means what um to set equal to something other than itself?

    like I could just put on every single question as the answer itself... it's not wrong of course but normally evaluate means not using the idenity property or whatever were x=x etc... no? to express the question in a different way???

    I think my anser is correct no like

    arcsin(4/5) does equal arcsin(4/5) but can you do that in your head I can't so I found the exact value... some people can evaluate the answer I got in their heads to get a numerical value... arcsin(4/5) really is difficult to get a numerical value from...

    l don't know you bring up a good point but I think I'm justified in writing that for my answer
  13. Aug 12, 2010 #12
    Anyways =O can you help my simplify
  14. Aug 12, 2010 #13
    Last edited by a moderator: May 4, 2017
  15. Aug 12, 2010 #14
    much like say what's
    2+1 and you put 2+1
    it's deffinatly not wrong lol but normally evaluate means express without using the identiy property what ever one that is lol

    but ya I would like to simplify the expression above further... I was wondering if maybe

    ln(pi)/pi could be simplifed some how...
  16. Aug 12, 2010 #15
    It's the reason I chose not to factor out the -i2 in the second expression I believe I can simplify ln(pi)/pi somehow also the fraction in the first term that is terms of the same base... to say cannot be simplifed... not buying it lol if it was

    5^5/(5^2 + 5^6)

    could be simplifed sense it's all in terms of the same base I just dont know how to simplify my answer...
  17. Aug 12, 2010 #16
    don't know if it really helps but

    ln(pi) = ln(-2 i ln(i) )
  18. Aug 12, 2010 #17
    Last edited by a moderator: May 4, 2017
  19. Aug 12, 2010 #18
    Well I'm trying to simplify and don't udnerstand why this isn't correct

    does not equal
    did i^2 = -1

    like if the quesiton was
    I could do
    which does equal

    Can somone explain why I can't do this with complex numbers?
  20. Aug 12, 2010 #19
    e = i^((-2 i)/pi)
  21. Aug 12, 2010 #20


    Staff: Mentor

    "To evaluate" just means find the value of. It would help if you gave the exact wording of the problem. If all you need to do is evaluate arcsin(4/5), you don't need all that work you show. arcsin(4/5) is about 53.13 degrees.
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