Easy math question

[hey_suburbia]

For some reason, I totally forget how to get the length of a side of an obtuse triangle. For instance this one:

I know the answer is 5.5, but what's the equation? Thanks

[DariusMonsef]

I'd use a ruler.

[hey_suburbia]

I did, that's why i know the answer.....

[McGiver]

as it's some random triangle:
use the law of cosine:
c²=a²+b²-2*a*b*cos(gamma)

[andr.in]

well I know that c² = a²+b²

[kirupa]

Are you given an angle by any chance? I think I remember learning a few years ago about a SAS (Side-Angle-Side) equation that may help you to find the missing 3rd side provided you know the angle between the two sides.

EDIT: here's a link that may help: http://www.andrews.edu/~calkins/math/webtexts/geom07.htm (http://www.andrews.edu/~calkins/math/webtexts/geom07.htm)

EDIT2: McGiver beat me to it :evil:

[jokun]

pythagreon's theorum (sp?)-

a²+b²=c²

so in this case, 4²+2²=?²

[senocular]

ruler... HA! http://senocular.com/smilies/laugh.gif

[McGiver]

c² = a²+b²
if it's an orthogonal triangle
( cos(90°) == 0 --> c²=a²+b²-2*a*b*cos(90°) == c² = a²+b² )

[jokun]

nm, others beat me too it,lol

[andr.in]

I just remembered c² = a²+b² wouldn't do since none of the corners is 90' :to:

[Beddoes]

its to do with the hypotenuse or however u spell it ,adjacent and opposite isn't it?
theres another way to do it but i cant remeber if u PM and want to know i'll look it up :D

[McGiver]

you can't get the other measures of a triangle if you only have 2 sides. there are infinite possible triangles with one side 2 units and an other one 4 units

[andr.in]

its to do with the hypotenuse or however u spell it no it's not... none of the corners is 90'.. that's why c² = a²+b² wouldn't do

[Beddoes]

im a bit tired but we are doing something like that in school i'll have a look and post here tomorrow :P

[hey_suburbia]

you can't get the other measures of a triangle if you only have 2 sides. there are infinite possible triangles with one side 2 units and an other one 4 units



I suppose that makes sense, because you can have the 4 unit line further out and the missing side will be different....

So, getting out the ruler IS the right answer after all...


right?

[McGiver]

right!

[jarviscares]

umm depends what kinda math. if it's pre-cal or calculus, ruler won't do. i've yet to use one... if it's applied, use your graphing calculator. otherwise... as far as i know you cannot solve unless you know an angle. cosine law won't work (don't know an angle) sine law wont work (don't know an angle), no right angle rules (trig) will work (dont' know if there is a right angle, assume no as it isn't labeled). it wasn't origionally inscribed in a circle... was it? cuz that's the only thing i can think of...


side note... a joke my math teacher told me....
Q: What does a Calculus grad student say?
A: What perks come with this $100k+ per year job?

Q: What does the Arts grad say?
A: Do you want fries with that :P


*edit...
o ya, and don't assume the answer is 5.5 if you only measured it... unless you're in like gr 5. cuz last i checked there is no rule in math saying that a triangle has to be drawn exactly right, as long as it's labeled.

in other words, an angle that truely measures 105 degrees could be given as 30 degrees... and you would have to assume 30 degrees. dont' go by measurements unless the question asks you to

and yet... i still wish i had taken art :(

[mlk]

as it's some random triangle:
use the law of cosine:
c²=a²+b²-2*a*b*cos(gamma)


Absolodutly =) - it's called the laws of Al-Kashi:

-a²=b²+c²-2bc*cos(A) [where A is the angle opposed to side 'a' in the triangle]
-b²=a²+c²-2ac*cos(B)
-c²=b²+a²-2ba*cos(C)
-a/sin(A) = b/sin(B) = c/sin(C)
-S=1/2*bc*sin(A) [where S is the surface of the triangle]
-S=1/2*ac*sin(B)
-S=1/2*ba*sin(C)

If my memory's correct...

edit: and for the problem, if you don't know any angles, then I gess you do have to use your ruler :trout:

[jokun]

I just remembered c² = a²+b² wouldn't do since none of the corners is 90' :to:

Whoops, that's right. :-/

[senocular]

Whoops, that's right. :-/


for those of you who missed it, the law of cosines IS pythagreon's theorum ... basically. pythagreon's theorum is the law of cosines reduced from a right angle in the triangle... McGiver tried to show this but I think it kind of blew past a lot of people (as it was just a big formula with crazy numbers and letters... whew!)

So in a way, pythagreon's theorum is more or less the answer, so long as you use the extended version which applies to non-right triangles... and that requires the extra baggage which is the law of cosines; formula provided above. :king:

[jarviscares]

hrrm.... unless this triangle is inscribed in a circle OR there is more information than was given... IT ISN'T SOLVEABLE (unless, as i stated, you're in gr. 7 and measuring will do.)

:D

[grinch]

Yeah why not just use a ruler? It makes life so much easier!

[senocular]

Im still all about the ruler ;)

[hificopymaster]

I don't own a ruler. If it ever comes down to it, I'll use my finger. I've only had to do it once thus far, but I got the right answer to within 1 decimal place. :)

[ScriptFlipper]

if I didn't have a ruler I'd make one in PS and print it out on paper :beam:
actually, I've done that once, since I couldn't find my ruler ;)

[morse]

pythagreon's theorum (sp?)-

a²+b²=c²

so in this case, 4²+2²=?²
That's only for right triangles.

[Beddoes]

yeh i had a look and to solve it you would have to have at least one angle...
Nt guys tho..

[jokun]

I suppose that makes sense, because you can have the 4 unit line further out and the missing side will be different....

So, getting out the ruler IS the right answer after all...


right?

Are you sure you have all the necessary info?

[pom]

Al-Kashi is the way to go, but you ABSOLUTELY need an angle in order to calculate that. So maybe the answer is not a ruler, but that thing you use to measure angles :-/

[jokun]

Al-Kashi is the way to go, but you ABSOLUTELY need an angle in order to calculate that. So maybe the answer is not a ruler, but that thing you use to measure angles :-/

Man, now we need a protractor, too?

[minimalistik]

Firstly its a scalene triangle which obtuse angles
Secondly, you cant use pythagoras theorem since its not a right angled triangle
Thirdly, you cannot use a ruler or protractor where in the case the question's diagram may not be to scale which of course they are not in book or else they would take a whole page wouldnt they?

You also need to supply the question with angles. You cant work out your question without any angles. In this case when you do have the angle, you can use the sine rule where two sides and the angle opposite one of them or cosine rule where 2 sides and an included angle. (a² = b² + c² - 2bc cos A)

Holla back when you do have the angle then its easy as 1-2-3.

edit: looks like someone else also asking maths questions around here :D

[jokun]

Firstly its a scalene triangle which obtuse angles
Secondly, you cant use pythagoras theorem since its not a right angled triangle
Thirdly, you cannot use a ruler or protractor where in the case the question's diagram may not be to scale which of course they are not in book or else they would take a whole page wouldnt they?


Yeh, I think we've pretty much talked these points to death already. It seems pretty obvious that we're either missing some info or some new, previously unheard of mathematical formula has recently been discovered that would allow us to solve this w/ basically no info. Hmmm...
*checks secret Illuminati math conspiracy guide book*
Nope, nothing in here.

Oh, and the protractor comment was just a joke.

[minimalistik]

Oh, and the protractor comment was just a joke.


lolz man i wasnt angry, i was only stating the points since i did it throughly before.
peace.

[jokun]

:P Man, you wanna fight? I just saw Spidey 2, n I'm lookin to open a can of Spidey sense on someone. :P

Sorry, I'm just pissy from lack of sleep. This is exactly why I never do well pulling all-nighters. I turn into a big jerk :sigh:

[minimalistik]

:D its ok man, i only get pissed off at night when maths is a pain. I got a test on friday on trig functions 2 so its more complex than sine/consine rules.

[jarviscares]

i think someone should lock this thread... the question is not solveable and there is no sense in people posting the same things over and over and over...

but i'm not a mod, so who am i to say :D