[" If "cos theta+cos phi=a" and "sin theta-sin phi=b],[" then ":2cos(theta+phi)=],[[" (a) "a^(2)-b^(2)-2," (b) "a^(2)-b^(2)+2],[" (c) "a^(2)+b^(2)+2," (d) "a^(2)+b^(2)-2]]