Circuit n00b: am I going in the right direction choosing my parts?

Hello, I am absolutely new to circuit building. I am designing a knee brace that measures the knee's three angles (flexion, rotation, and side-to-side swing) using TWO 3-axis gyros from sparkfun. I plan to have the sensors input into an Arduino BT (bc I need Bluetooth later).

Both gyros give me the velocity at each time point so I plan to divide the velocity of both sensors by the time step to get the change in angle. It will find the 3 included angles between sensors (dot product rule) and compare this to the zero position angles (set when Arduino is plugged in). Then, it will tell the person if they are standing 'too crooked' by outputting to a vibration motor + red/green LED AND show the angles real time on the computer screen (yay Labview!).

Question: do Arduinos have the cosine function built in? (otherwise I could calculate this manually) And does this seem like the correct direction? THANKS!

do Arduinos have the cosine function built in?

Yes, along with sin, tan, and most of the others.