Which of these is equal to cos 19°? a) sin 71° b) sin 81° c) cos 71° d) cos 81°

If we draw a right triangle with an angle of 19° in it, then the other angle is  90° - 19° = 71° (because all angles in a triangle add up to 180°).see attached diagram. let y be the opposite side to 19°, let x be its adjacent, let r be hypotenuse.cosine is adjacent/hypotenuse, so  for 71° to get the same ratio, we use the sine of that angle.since these are the same thing, the answer is a