calculus and analysis – Mathematica outputs sin(Pi*t) as Pi*sin(t)

Im very new to Mathematica, so spare me if there is a very trivial solution to this.

In(55):= c := cos (Pi*t)
         d := sin (Pi*t)

Out(57)= cos(Pi*t)+Pi*sin(t)

Why does Mathematica output sin(Pi * t) as Pi*sin(t)? The cosine gets a correct output but the sine for some reason doesn’t.