Question 744305
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).<P>
cos(A+B) = cosA*cosB - sinA*sinB
cos(225) = cos(180+45) = cos(180)*cos(45) - sin(180)*sin(45) = -1*{{{sqrt(2)/2}}} - 0*{{{sqrt(2)/2}}} = {{{-sqrt(2)/2}}}