SOLUTION: Using the angle difference identity to find the exact value of cos225. I have come up with (-sqrt2)/2. Is this correct? Please help!

Algebra ->  Trigonometry-basics -> SOLUTION: Using the angle difference identity to find the exact value of cos225. I have come up with (-sqrt2)/2. Is this correct? Please help!       Log On


   

Question 744305: Using the angle difference identity to find the exact value of cos225. I have come up with (-sqrt2)/2. Is this correct? Please help!
Answer by fcabanski(1391) About Me  (Show Source):
You can put this solution on YOUR website!
Use the sum and difference identities to express the trig functions in terms of the functions most people memorize (30, 45, 60, 90, 180, 270, 360).


cos(A+B) = cosA*cosB - sinA*sinB
cos(225) = cos(180+45) = cos(180)*cos(45) - sin(180)*sin(45) = -1*sqrt%282%29%2F2 - 0*sqrt%282%29%2F2 = -sqrt%282%29%2F2