1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Finding the value of A and B (Another problem)

  1. Nov 28, 2015 #1
    Hello experts,
    I have attached the figure below kindly see it. And here is my calculation,
    Using law of sine,
    from figure we can say that α+β=120°
    and β=120°-α
    and sin(β)/sin(α)=PA/PB=3/40=0.075

    so,
    sin(120°-α)=sin(120°)cos(α)-cos(120°)sin(α)
    since, sin(β)=sin(120°-α)
    so,
    sin(β)=sin(120°)cos(α)-cos(120°)sin(α)
    dividing both sides by sin(α)
    so,
    sin(β)/sin(α)=sin(120°)cos(α)/sin(α)-cos(120°)sin(α)/sin(α)
    sin(β)/sin(α)=sin(120°)cos(α)/sin(α)-cos(120°)
    sin(β)/sin(α)=0.866*cos(α)/sin(α)-(-0.5)
    3/40=0.866*cot(α)+0.5
    0.075=0.866*cot(α)+0.5
    -0.425=0.866*cot(α)
    cot(α)=-0.425/0.866
    cot(α)=-0.49
    α=-63.86°

    What is this? Why α is -ve? Where I have gone wrong? From figure we can clearly see that it is more than 90°. But calculation says it is -63.86°.

    According to the book point A is at -3.70 and B is at -53.35. But I am not getting my angle correct. HElp Capture.PNG
     
  2. jcsd
  3. Nov 28, 2015 #2

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    What other solutions are there to cot(α)=-0.49?
     
  4. Nov 28, 2015 #3

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Nope, I don't see any figure.
     
  5. Nov 29, 2015 #4
    Other solutions mean?
     
  6. Nov 29, 2015 #5
    Don't you see the figure of triangle?
     

    Attached Files:

  7. Nov 29, 2015 #6

    haruspex

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    There's more than one angle in a 360 degree range with a cotangent equal to a given value. For a positive value, there will be one in the first quadrant and one in the third. Here you have a negative value.
     
  8. Nov 29, 2015 #7
    No. I dont understand your point of view about more than one angle. Please elaborate with example.
     
  9. Nov 29, 2015 #8

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    I do now that you have posted the figure.
     
  10. Nov 29, 2015 #9

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    This figure shows the geometric representation of the cotangent:

    http://www.regentsprep.org/regents/math/algtrig/att5/unitcircletriglabel.jpg
    This particular figure shows the cotangent for an angle θ which is in the first quadrant. Like its cousin the tangent, cotangent is positive for angles in the first and third quadrants, but negative for angles in the second and fourth quadrants.

    You have to figure out how your result relates to your original triangle.
     
  11. Nov 29, 2015 #10
  12. Nov 29, 2015 #11

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    You have different triangles.

    In the first problem, angle α was less than 90°. In the current problem, angle α is greater than 90°. Don't you think that makes a difference in the value of cot(α)?
     
  13. Nov 29, 2015 #12
    Yes, you are right. I have already observed it. But I my calculation is for α so why my α is not correct?
    If 180-63.86=116.14

    This is greater than 90. And from figure we can see that α is also greater than 90.

    If 116.14 is the value for what I am looking for then I don't that understand that why this time I should subtract it from 180. why not in the last question I did this.

    Much confusion :'(
     
  14. Nov 29, 2015 #13

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    I don't think anyone is suggesting that angle α be subtracted from 180°. It's just that your original value of α = -63.86°. What you must do is reconcile this result with the triangle which shows that α > 90°. :wink:
     
  15. Nov 29, 2015 #14
    No. I really don't understand.

    I have calculated the value of α, then what should be further can I do with it?

    And you are true no one suggested me to subtract it. But I don't see any further solution.
     
  16. Nov 29, 2015 #15

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    What if you added angle α algebraically to 180°? Wouldn't that give you a result which satisfies the triangle? When figuring angles, references are important.
     
  17. Nov 29, 2015 #16
    I don't want to lost in the maze. Kindly let me follow the light not to the darkness.

    I am closing thread. I don't find any solution to my question.

    Thank you for your time.
     
  18. Nov 29, 2015 #17

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    That's too bad. You really should review trigonometry to get a better handle on it. Trig pops up in all kinds of math and engineering.
     
  19. Nov 29, 2015 #18
    I don't have time to review it. I have exam tomorrow on 30th November, 2015. 3 years ago we have trigonometry as a subject. After 3 years it comes again and in this 3 years we only have integration more than trigonometry.
     
  20. Nov 29, 2015 #19

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    Doesn't matter. Trig is one of those things which you use frequently. It doesn't matter if you studied it 3 years ago or 40 years ago like I did. You have to keep on top of it or you could risk blowing an exam. :frown:
     
  21. Nov 29, 2015 #20
    OK just solve me this problem to the value of α then further I will try on my own. But kindly find the exact value for me of α
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Finding the value of A and B (Another problem)
  1. Another Problem (Replies: 3)

  2. Another problem (Replies: 8)

Loading...