# Thread: Need help inputting law of cosines on ti83+

1. ## Need help inputting law of cosines on ti83+

Solve:

Ok, so I use the law of cosines, correct?

$\displaystyle cos A = b^2+c^2-a^2/2bc$

Now, when I input this into my ti83+:

$\displaystyle 2017^2+2938^2-2506^2/2(2017x2938)$

I get -1.86075644e13

So, next I am supposed to multiply by the inverse cosine, right? But -1.8 is obviously not right, so what am I doing wrong?

2. Originally Posted by coasterguy
Solve:

Ok, so I use the law of cosines, correct?

$\displaystyle cos A = b^2+c^2-a^2/2bc$

Now, when I input this into my ti83+:

$\displaystyle 2017^2+2938^2-2506^2/2(2017x2938)$

I get -1.86075644e13

So, next I am supposed to multiply by the inverse cosine, right? But -1.8 is obviously not right, so what am I doing wrong?
check that your calculator is in degree mode. here's how it should look on your home screen ...

3. Originally Posted by skeeter
check that your calculator is in degree mode. here's how it should look on your home screen ...

Thank you!