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Simple trig question

  1. Oct 7, 2012 #1
    If I have a right triangle and I know the hyp and the length of the opposite side of the angle I want then how do I find that angle? For instance: hyp = .9m and side opposite the angle I want = 1.5m. I tried dividing 1.5/.9 = 1.7 then I thought all i had to do was take the inverse sin of 1.7 and it would give me my angle but my calc gives me an error... So im confused, how do I get the angle i want?
  2. jcsd
  3. Oct 7, 2012 #2
    Sine oscillates between -1 and 1 so you can't take the inverse sine of 1.7, right? You should probably draw a picture and make sure you've got the relationships right.
  4. Oct 7, 2012 #3
    I figured out that it's not going to work like that because im asking for a value greater than 1... ok so I acted to brashly and asked for help before I thought my question through sorry guys ignore this post.
  5. Oct 7, 2012 #4
    Anyway, the hypotenuse of a right triangle can't be never shorter than any cathetus, so you must have wrong lengths.
  6. Oct 7, 2012 #5
    Okay so here is the problem. If I have a circumference of .9m and I want the radius of the circle then I just divide .9 by 2pi right? Which according to my calculator = 1.5 m... However, when I put this same problem into Microsoft mathematics it says r = .14 When I look at the solution steps it says something about how dividing by 2pi undoes the multiplication of 2pi... I guess I am missing something here.
  7. Oct 7, 2012 #6
    Plus 1 for cathetus. I've only ever heard the word "leg" in this context. Good knowledge! And welcome to the forum.

    That can't be right. One little math trick is to always compare calculator results to common sense. That's a lifesaver on tests. As you work, continually make common-sense estimates and see if your numbers are in the ballpark.

    Pi is about 3, right? That's close enough for the moment. So what's 2pi? It's about 6. And if I take .9 and divide by 6, how can the answer be 1.5? If I start with .9, which is a little less than 1; and I divide it into 6 pieces; then each piece needs to be way less than 1. Can't be greater than 1. So right here if you're thinking about this as you go, you'd know you made a mistake.

    I apologize for being a dinosaur here ... but you should put down that calculator. You're punching numbers in as a substitute for thinking about what's going on. Easy to do. But it can lead you astray. Better to just work this problem out on paper.

    By the way, 9 divided by 6 is 1.5. You forgot the decimal point. But if you develop the habit of doing reality checks as you work, you'll avoid these kinds of errors on tests.
    Last edited: Oct 7, 2012
  8. Oct 7, 2012 #7


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    Then get a new calculator or reread the manual! A small number divided by a larger number (2pi is larger than 6) cannot be larger than 1. .9 divided by [itex]2\pi[/itex] is about .14. Surely you aren't under the impression that [itex]\pi[/itex]= 0.314.... but that would be my best guess- that you have drop a factor of 10.

    Uh, basic arithmetic? When you learn how to divide, in the second or third grade, you should learn that division is the reverse of multiplication.
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