think of the right triangle.
Also Pythagorus' theorem:
where a is the hypotenuse, and b and c are adjacent and opposite sides respectively.
Essentially the same: think of as representing a right triangle with "near side" of length 8 and "opposite side" of length 1 so that = "near side"/"opposite side"= 8. Use the Pythagorean theorem to find the hypotenuse and then use the formulas arze gives.