13.
If the Fourier transform F[δ(x - a)] = exp (-i2πv a), then F-1(cos 2π av) will correspond to

14.
The solutions to the differential equation \[\frac{{dy}}{{dx}} = - \frac{x}{{y + 1}}\]   are a family of

15.
The Laplace transform of f(t) = sin πt is $$F\left( s \right) = \frac{\pi }{{{s^2}\left( {{s^2} + {\pi ^2}} \right)}},\,s > 0.$$     Therefore, the Laplace transform of t sin πt is

16.
A finite wave train, of an unspecified nature, propagates along the positive X-axis with a constant speed v and without any change of shape. The differential equation among the four listed below, whose solution it must be, is

19.
Which of the following vectors is orthogonal to the vector \[\left( {a\hat i + b\hat j} \right),\]   where a and b (a ≠ b) are constants, and \[{\hat i}\] and \[{\hat j}\] are unit orthogonal vectors?

20.
The two vectors \[\overrightarrow P = i,\,\overrightarrow Q = \frac{{i + j}}{{\sqrt 2 }}\]     are