The right triangle above has side lengths a and b such that 0 < b < a. One of its angles measures arctan(a/b). What is cos(arctan(a/b))?
If the angle is arctan a/b then it is the angle in the upper left corner because tan = opp/adj
the cos of that angle is adj/ hyp = b / (sqrt(a2+b2) (because hypot in rt triangle = sqrt(a^2 + b^2) AND COS = adj / hyp )