Final answer:
To find an expression equal to cos 24º using a cofunction, use the fact that the cofunction of an angle is equal to the value of the trigonometric function applied to the complementary angle.
Step-by-step explanation:
To find an expression equal to cos 24º using a cofunction, we need to use the fact that the cofunction of an angle is equal to the value of the trigonometric function applied to the complementary angle. The complement of 24º is 90º - 24º = 66º. Therefore, the expression equal to cos 24º is option a) sin(90° - 24°).