1a/
Given x(t) find X(f)
$ x_(t) \,\!= \cos(\frac{\pi}{2})rect(\frac{t}{2}) \quad (1) $
Using the convolution property
$ X_(f) = \mathcal{F} (cos(\frac{\pi t}{2}))* \mathcal{F}(rect(\frac{t}{2})) $
where
$ \mathcal{F} (cos(\frac{\pi t}{2})) = \frac{1}{2} [\delta(f - \frac{1}{4}) + \delta(f + \frac{1}{4})] $
and
$ \mathcal{F}(rect(\frac{t}{2})) = 2 sinc( 2 f) $
substituting the known transforms into $ \quad (1) $
$ X_(f) = \frac{1}{2} [\delta(f - \frac{1}{4}) + \delta(f + \frac{1}{4})] * 2 sinc( 2 f) $
Evaluating the statement ( using sifting )
$ X_(f) = sinc(2 (f - \frac{1}{4}) + sinc( 2(f+\frac{1}{4})) $
