Question 19 6 0/1 pt O 100 # 99 0 Details Use a half angle formula to fill in the blanks in the identity below: (cos (2x))' = cos ( I) Question Help: Message instructor O Post to forum Submit Question Question 20 0/1 pt O 100 # 99 0 Details For cos & cos 3x - sin a sin 3x = 0, use an addition or subtraction formula to simplify the equation and then find all solutions of the equation in the interval [0, ). The answer is $1 = 2 = 1 3 = and I4 = with $1