# Thread: Law of Cosines, help!

1. ## Law of Cosines, help!

Given these dimensions, I have to find the remaining angles.
I know you're supposed to use the law of cosines which would be a²=b²+c²-2bcCosA

So plugging that in I have
a²=2337²+5168²-2(2337(5168))CosA

Is this correct? I'm not sure how to get rid of the CosA.

2. Originally Posted by alexcross
Given these dimensions, I have to find the remaining angles.
I know you're supposed to use the law of cosines which would be a²=b²+c²-2bcCosA

So plugging that in I have
a²=2337²+5168²-2(2337(5168))CosA

Is this correct? I'm not sure how to get rid of the CosA.

get rid of cos(A) ??? you were given the value of angle A ...

$\displaystyle a = \sqrt{5168^2 + 2337^2 - 2(5168)(2337)\cos(137.2^{\circ})}$

punch it out on your calculator.

3. Oh wow duh haha I completely forgot to plug in A. Thank you!

I have a scientific calculator, not graphing. So whenever i press the Cos button at the end for the 137.2, it does Cos for the entire equation. How do I type it to only use the Cos function on the 137.2?

I think I did it, is the answer 6405.5835?

4. no ... you should get about 7063

5. Well crap I don't know what I'm doing wrong. Why doesn't the Pythagorean Theorem work for finding a?

6. ??The Pythagorean theorem only applies to right triangles. The cosine law essentially is the Pythagorean theorem extended to other triangles.

If your calculator is one of those where you put in the number first, then press the "cos" key, try doing that first, then saving the value to memory and using that in the formula. If your calculator has no memory, input the angle, then press "cos", multiply by b and -c, then add the squares.

Oh, and be sure your calculator is in "degree mode", not radian!