code for above diagrams

> grph:=plot(-9.8/2*t^2+80,t=0..4,linestyle=3):

> f:=t-> -9.8/2*t^2 +80:

> Digits:=4: pts:=[seq([t,f(t)],t=0..4)]: Digits:=10:

> PTS:=plot(pts,style=POINT, thickness=3,color=black,symbol=circle):
plots[display]([grph,PTS]);