The law of cosines to solve a triangle?

Question

Hello,
I am given the formula c^2=a^2+b^2-2ab. The text shows this is how you set it up... cosC=965^2+876^2-1240^2/
2(965)(876).
The answer according to the solution
manual is .09522855.
When I do the math I get -64989569339

What am I doing wrong? How do I get the
answer the book has from the numbers
and formula given?

Thanks for your help.

Answer 1

the actual formula is

c^2 = a^2 + b^2 - 2ab(cos(C))

2ab(cosC) + c^2 = a^2 + b^2
2ab(cosC) = a^2 + b^2 - c^2
cosC = (a^2 + b^2 - c^2)/(2ab)

cosC = (965^2 + 876^2 - 1240^2)/(2 * 965 * 876)
cosC = (931225 + 767376 - 1537600)/(1690680)

cosC = (161001/1690680)
cosC = .09522854709

rounded

cosC = .09522855

Maybe by looking at what i did, you can find out where you went wrong.

Answer 2

What you ahve typed in is not Law of cosines, you're missing cos. Law of cosines is:
c^2=a^2+b^2-2abcosC

I've never seen such huge numbers of a, b, and c for a problem like this before. I think you may be missing some decimal points. Make sure you use the correct formula above for your calcuations.

Answer 3

I am almost positive that your calculations are just wrong, the answer accourding to the solution is just Cos(c) = .095228547, the numbers you have provided are correct, check you parenthesis
you should get (161001)/ (2*965*876) which does equal.09522855

http://hyperphysics.phy-astr.gsu.edu/hbase/lcos.html

Answer 4

The formula is : c^2=a^2 + b^2 - (2ab)*(cos c)

The angle that you would take the cosine of is the angle across from side c

Answer 5

You divide all of c^2 - a^2 - b^2 by 2ab. The way you wrote it, you only divided c^2 by 2ab.

Answer 6

i dont know what type of calculator you have, but make sure its on "degree" in stead of "radian" mode when you press the mode button because that would be messing you up.