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A trig question without calculator

  1. Jan 15, 2012 #1
    1. The problem statement, all variables and given/known data
    sinx=-1/2 how do I find x without using a calculator?

    2. Relevant equations

    3. The attempt at a solution
    I really don't have any idea.
  2. jcsd
  3. Jan 15, 2012 #2
    Last edited: Jan 15, 2012
  4. Jan 15, 2012 #3
    You are wrong. sin(x) is between 1 and -1.

    To the original poster: Use the unit circle to help you solve this.
  5. Jan 15, 2012 #4

    Yes i was wrong. I've corrected it.
  6. Jan 15, 2012 #5


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    Staff: Mentor

    Use a calculator, memorize the result, then whenever you again encounter the problem you can recall the result from memory and without using a calculator. It relates to a triangle that you should commit to memory--memorize its sides and angles. Having done this, you'll probably realize that you should have been able to work it out without using the calculator, had you known.
  7. Jan 15, 2012 #6
  8. Jan 15, 2012 #7
    Thanks. I'm going to memorize some common trig values as advised.
  9. Jan 15, 2012 #8


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    Science Advisor
    Homework Helper

    You are better off picturing the 30-60-90 and 45-45-90 triangles than memorizing the trig values. They'll stay in your mind a lot longer than the numbers, and it's pretty easy to derive the trig values from the picture and the Pythagorean theorem.
  10. Jan 16, 2012 #9
    there's two right angled triangles that I remember for common trig values

    one has two 45 degree angles, two sides of length 1 and hypotenuse of length sqrt(2)
    the other has 30 degree and 60 degree angles , with side lengths 1 sqrt(3) and hypotenuse 2

    apart from those I usually just use a taylor series about some point that I know the value of for a quick approximation
  11. Jan 16, 2012 #10
    Draw a right angle triangle with the hypotenuse twice the length of the opposite, measure the angle with a protractor.
  12. Jan 16, 2012 #11
    Dick has good advice. I wouldn't suggest memorizing the number based on what the calculator told you, unless this is the very last math class you are ever going to take and you don't forsee yourself ever having to or wanting to do a geometric/trig problem after your current class...Otherwise, you should understand how the number is calculated by studying and then commiting to memory the triangles given and also memorizing the unit circle would be helpful too.
  13. Jan 17, 2012 #12


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    Staff: Mentor

    I suggested that you memorize some common triangles, then you can work out a few useful trig values from the triangles.
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