Find the positive value of $ k$ for which $ \int_0^{\frac {\pi}{2}} |\cos x \minus{} kx|\ dx$ is minimized.
1. We start by considering the integral \( \int_0^{\frac{\pi}{2}} |\cos x - kx| \, dx \). To handle the absolute value, we need to find the point \( a \in \left(0, \frac{\pi}{2}\right) \) where \( \cos a = ka \). This point \( a \) is where the expression inside the absolute value changes sign.

2. We split the integral at \( a \):
   \[
   \int_0^{\frac{\pi}{2}} |\cos x - kx| \, dx = \int_0^a (\cos x - kx) \, dx + \int_a^{\frac{\pi}{2}} (kx - \cos x) \, dx
   \]

3. We compute each part of the integral separately:
   \[
   \int_0^a (\cos x - kx) \, dx = \int_0^a \cos x \, dx - k \int_0^a x \, dx
   \]
   \[
   = \sin a - k \frac{a^2}{2}
   \]

   \[
   \int_a^{\frac{\pi}{2}} (kx - \cos x) \, dx = k \int_a^{\frac{\pi}{2}} x \, dx - \int_a^{\frac{\pi}{2}} \cos x \, dx
   \]
   \[
   = k \left[ \frac{x^2}{2} \right]_a^{\frac{\pi}{2}} - \left[ \sin x \right]_a^{\frac{\pi}{2}}
   \]
   \[
   = k \left( \frac{\pi^2}{8} - \frac{a^2}{2} \right) - (1 - \sin a)
   \]

4. Combining these results, we get:
   \[
   \int_0^{\frac{\pi}{2}} |\cos x - kx| \, dx = \left( \sin a - k \frac{a^2}{2} \right) + \left( k \frac{\pi^2}{8} - k \frac{a^2}{2} - 1 + \sin a \right)
   \]
   \[
   = 2 \sin a - k a^2 + k \frac{\pi^2}{8} - 1
   \]

5. Given \( \cos a = ka \), we substitute \( k = \frac{\cos a}{a} \) into the expression:
   \[
   f(a) = 2 \sin a - a \cos a + \frac{\pi^2}{8} \cdot \frac{\cos a}{a} - 1
   \]
   \[
   = 2 \sin a - a \cos a + \frac{\pi^2 \cos a}{8a} - 1
   \]

6. To minimize \( f(a) \), we take the derivative \( f'(a) \) and set it to zero:
   \[
   f'(a) = \cos a + a \sin a - \frac{\pi^2}{8a^2} (\cos a + a \sin a)
   \]
   \[
   = (\cos a + a \sin a) \left( 1 - \frac{\pi^2}{8a^2} \right)
   \]

7. Setting \( f'(a) = 0 \), we get:
   \[
   \cos a + a \sin a = 0 \quad \text{or} \quad 1 - \frac{\pi^2}{8a^2} = 0
   \]
   \[
   1 - \frac{\pi^2}{8a^2} = 0 \implies a^2 = \frac{\pi^2}{8} \implies a = \frac{\pi}{2\sqrt{2}}
   \]

8. Substituting \( a = \frac{\pi}{2\sqrt{2}} \) back into \( k = \frac{\cos a}{a} \):
   \[
   k = \frac{\cos \left( \frac{\pi}{2\sqrt{2}} \right)}{\frac{\pi}{2\sqrt{2}}} = \frac{2\sqrt{2}}{\pi} \cos \left( \frac{\pi}{2\sqrt{2}} \right)
   \]

The final answer is \( \boxed{ k = \frac{2\sqrt{2}}{\pi} \cos \left( \frac{\pi}{2\sqrt{2}} \right) } \)