Re: Question about periodic signals
You did not study it on the course? Or the teacher did not provide book for it?
A function is periodic if f(z)=f(z+T) with z being a complex variable.
To help you out, for the exponentials use Euler's formula.
For the c) function, I will help you with this.
Consider f(x)=sin(x)/x = sinc(x) (in is not exactly what you have but works since you multiply by a constant)
Let assume it is periodic => f(x)=f(x+T). T is the period which can be took for whatever interval you want, I will take for the inverval [0,T] meaning f(0)=f(0+T) for x=0.
Ok, so lets start..
f(0)=f(T) <=> 1=sin(T)/T <=> sin(T)=T => that equation has one and only one solution for T=0, meaning the function is periodic with period T=0 meaning it is not periodic.