# Math Help - cos-1

1. ## cos-1

I try to find X from the cosine formula but it doesn't sound right

12sq = 8 sq + 5sq - 2x 8x 5cosX
144 = 64 + 25 - 80cosX
55 = -80cosX
x= cos^-1(-55/80)

I keep getting 2.328837092

2. Well, when you get the correct answer, you should quit. Why are you still streesing over it?

Would you be happier with $\pi - acos\left(\frac{11}{16}\right)$

Maybe you were expecting the answer in degrees? 133.433 deg

3. how did you get 133.433 deg?

"In mathematics, you don't understand things. You just get used to them." -- Johann von Neumann

4. Originally Posted by terminator
how did you get 133.433 deg?
using degree mode in your calculator ...

$\displaystyle 180 - \arccos\left(\frac{11}{16}\right) \approx 133^\circ$

5. Originally Posted by terminator
I try to find X from the cosine formula but it doesn't sound right

12sq = 8 sq + 5sq - 2x 8x 5cosX
144 = 64 + 25 - 80cosX
55 = -80cosX
x= cos^-1(-55/80)

I keep getting 2.328837092
Since $\pi= 3.14159...$ radians corresponds to 180 degrees, 2.3288 radians= 2.3288 (180/3.14159)= 133.433... degrees.