A Trig Question

rarisgod · #1 09-18-2010, 08:00 PM

A triangle has three sides, 1.6m, 1.9m and 0.32m.
What are the three angles within the triangle?

We have the answer, but apparently my math skills aren't working for such a basic question.

Could someone do this question and provide your steps as well?

I thought I knew how to do this, but apparently my answer doesn't match.

oldrocker · #2 09-18-2010, 08:09 PM

Not sure - but I bet they add up to 180

rarisgod · #3 09-18-2010, 08:12 PM

Quote:

Originally Posted by oldrocker

Not sure - but I bet they add up to 180

Yes of course, but the problem is that we aren't given any angles to start with.

My thought was to plug all the given sides into the cosine law, and do inverse cos at the end to find the angle (then repeat for the other angles) however our answers always end up wrong.

I've been through 2 university calculus courses, yet this just isn't working for me. (trying to help someone)

timmus · #4 09-18-2010, 08:23 PM

18.612 deg
157.727 deg
3.66 deg

using law of Cosines

A² = B² + C² −2(B)(C)cos(angle 1)
B² = A² + C² −2(A)(C)cos(angle 2)
C² = A² + B² −2(A)(B)cos(angle 3)

A,B and C being the lengths of the sides

**nutdog** · #5 09-18-2010, 08:28 PM

http://www.teacherschoice.com.au/mat...e_trig_sss.htm

timmus · #6 09-18-2010, 08:31 PM

That works too...

rarisgod · #7 09-18-2010, 08:32 PM

Quote:

Originally Posted by timmus

18.612 deg
157.727 deg
3.66 deg

using law of Cosines

A² = B² + C² −2(B)(C)cos(angle 1)
B² = A² + C² −2(A)(C)cos(angle 2)
C² = A² + B² −2(A)(B)cos(angle 3)

A,B and C being the lengths of the sides

That is exactly what I did, I just felt that 3.66 was a bit far from the given solution of 3.3.

The given answers are:
3.3
158.1
19.5

See? We got the same answers as you, so why are their given solutions all off? Would the professor do this on purpose, or how else could the given solutions be that far off?

Thanks a lot for helping out!

**nutdog** · #8 09-18-2010, 08:33 PM

I'm too lazy to do it. And it would give me really bad flashbacks.

timmus · #9 09-18-2010, 08:36 PM

ah yes...me too..too many years since I got my EE. We did all this stuff on a slide rule back then. Boy, I'm dating myself now

Marco A. Mayer · #10 09-18-2010, 08:36 PM

I love math, but trigonometry is the single worst thing to learn in school. I just can't stand it.

**nutdog** · #11 09-18-2010, 08:39 PM

Quote:

Originally Posted by rarisgod

That is exactly what I did, I just felt that 3.66 was a bit far from the given solution of 3.3.

The given answers are:
3.3
158.1
19.5

See? We got the same answers as you, so why are their given solutions all off? Would the professor do this on purpose, or how else could the given solutions be that far off?

Because there is no truth. One answer is as good as another. And I see you're in Canada. Maybe that's it.

timmus · #12 09-18-2010, 08:39 PM

Quote:

Originally Posted by rarisgod

That is exactly what I did, I just felt that 3.66 was a bit far from the given solution of 3.3.

The given answers are:
3.3
158.1
19.5

See? We got the same answers as you, so why are their given solutions all off? Would the professor do this on purpose, or how else could the given solutions be that far off?

maybe the prof is trying to throw you off to prove him wrong...I had a circuits analyses prof do that a few times, just to see if we were on our toes...

Math is either right or wrong...if you and I came up with the same answer irrespective of each other, then I would say the prof is wrong

timmus · #13 09-18-2010, 08:42 PM

Quote:

Originally Posted by Marco A. Mayer

I love math, but trigonometry is the single worst thing to learn in school. I just can't stand it.

I must be a sucker for pain...I loved trig. I hated probability and statistics...I used to call it "unmath". To many variables and unknowns for me to be called "real" math...

**nutdog** · #14 09-18-2010, 08:47 PM

trig OK. linear algebra--aaaaaaaaahhhhh!

rarisgod · #15 09-18-2010, 08:51 PM

Quote:

Originally Posted by timmus

maybe the prof is trying to throw you off to prove him wrong...I had a circuits analyses prof do that a few times, just to see if we were on our toes...

Math is either right or wrong...if you and I came up with the same answer irrespective of each other, then I would say the prof is wrong

This is possible. I think maybe he might just be trying to make them 100% confident in their math skills so they won't feel the need to check their answers.

Personally I dont agree with that, you're trying to learn the material and I've always found that having the answers present allows me to confirm that I'm actually learning and not just guessing. But hey it's not my program.

rarisgod · #16 09-18-2010, 08:54 PM

Maybe someone could give my other question a shot?
http://www.talkbass.com/forum/showth...=1#post9727367

Thunderscreech · #17 09-18-2010, 09:35 PM

Draw a picture of it. But scale it down to centimeters or something. I could do it in AutoCAD if I had AutoCAD at my fingertips, but I'm on my MacBook.

**bassybill** · #18 09-19-2010, 12:53 PM

http://www.analyzemath.com/Geometry_...alculator.html

Needs Java to work.

funkydjembe · #19 09-19-2010, 01:30 PM

all angles=180 degrees. 1.6+1.9+0.32=3.82
180 divided by 3.82= 47.1020419blah
47.1 times 0.32=15.08
47.1 times 1.6= 75.4
47.1 times 1.9=89.53

took a quick chance on this,,,any doubts?

**bassybill** · #20 09-19-2010, 01:42 PM

Quote:

Originally Posted by funkydjembe

all angles=180 degrees. 1.6+1.9+0.32=3.82
180 divided by 3.82= 47.1020419blah
47.1 times 0.32=15.08
47.1 times 1.6= 75.4
47.1 times 1.9=89.53

took a quick chance on this,,,any doubts?

That won't work - the ratio of the angles is not just the ratio of the sides. Consider the pythagorean triad 3, 4, 5. The side opposite the right angle would be exactly half the perimeter if your method worked; clearly this is not the case.

Given 3 sides of a triangle, you use the cosine rule to work out the biggest angle and then the sine rule to get one of the other two angles. Then get the final angle by subtraction from 180.