When a body slides down from rest along a smooth inclined plane making an angle of `30^@` with the horizontal, it takes time T. When the same body slides down from the rest along a rough inclined plane making the same angle and through the same distance, it takes time aT, where a is a constant greater than 1. The co-efficient of friction between the body and the rough plane is `1/sqrtx ((a^2 - 1)/a^2)` where x = ________.