Getting smooth curve with gnuplot

I'm not getting smooth curve on gnuplot.

This is my code:

set style line 3 lc rgb '#09ad00' lt 1 lw 1.5 #green
set style line 1 lc rgb '#0060ad' lt 1 lw 2  #blue 
set style line 2 lc rgb '#dd181f' lt 1 lw 2  #red
plot [-1:1] f1(x) with line ls 3,f2(x) with line ls 1,f1(x)+f2(x) with line ls 2

I'm getting this plot

while I'm expecting this type of curve