The families of curves represented by the solution of the equation

$${{dy} \over {dx}} = - {\left( {{x \over y}} \right)^n}$$

for n = –1 and n = 1 respectively, are

The strategies that the company _______ to sell its products ________ house-to-house marketing.

The boat arrived _______ dawn.

Five different books (P, Q, R, S, T) are to be arranged on a shelf. The books R and S are to be arranged first and second respectively from right side of the shelf. The number of different orders in which P, Q and T may be arranged is ________.