# Math Help - Law of Cosines problem

1. ## Law of Cosines problem

Hi,

I am asked to solve the triangle ABC with the given information:
A=115 deg. b=46.8 c=51.3

First I solved for side "a" and discovered it be to "82.8." Subsequent to this, I tried to find out what angle "B" was. Setting up the law of cosines equation (b^2=a^2+c^2-2acCosine B) and plugged in the values as follows:

46.8^2=82.8^2+51.3^2-2(82.8)(51.3)Cos B

Simplifying and dividing both sides of the equation, I acquire 2.21 and that was what cos B was equal to. To find that number in degrees, I clicked the cos inverse button and, to my dismay, received a prompt error message. I have tried this problem over and over. Where am I making this "foolish" mistake that I can not seem see?

2. Your a is correct. However, your 2.21 is incorrect. You're not correctly solving for cos(B). Try that again. You should get something between -1 and +1.