Math Help - Fraction inside a square root. need some help

haven't taken math in a while and I'm taking a trigonometry class. I believe i learned this in algebra but i can't remember how its done.

cos = +/-√1-225/289

(basically i don't understand how to do the square root part of this. its the square root of 1- 225 over 289.

The answer for this is = +/- 8/17

**no clue how they got 8/17.
thx and godbless if anybody can help me

http://i48.photobucket.com/albums/f2...799/sqroot.jpg