reset
set border
set xtics 1
set ytics 1
set grid
set xzeroaxis lt 9 lw 3
set yzeroaxis lt 9 lw 3
set style line 1 lt -1 lw 4
set style line 2 lt 5 lw 3
set style line 3 lt 12 lw 3
set xrange [-10:10]
set yrange [-5:18]
a=5
b=9
f(x)=sqrt((-b**2)*((1-x**2)/a**2))
plot f(x) ls 1