. Find radian measure of an angle of $90^{\circ}$. Enter the radian to be between 0 and (Enter an exact answer. Use symbolic notation and fractions where needed.) \[ r= \]

Step 1 ：Given an angle of 90 degrees, we are asked to find its radian measure.

Step 2 ：We know that \(1^{\circ} = \frac{\pi}{180}\) radians.

Step 3 ：So, to convert 90 degrees into radians, we multiply 90 by \(\frac{\pi}{180}\).

Step 4 ：This gives us a radian measure of \(\frac{\pi}{2}\).

Step 5 ：Final Answer: The radian measure of an angle of $90^{\circ}$ is \(\boxed{\frac{\pi}{2}}\).