smc

The discriminant of a quadratic is $$\Delta =b^2-4ac=(2b{)}^2-4ac=4b^2-4ac=0.$$ We know that $b^2=ac,$ or $\frac{b}{a}=\frac{c}{b}$ so $a,b,c$ form a geometric progression. $\boxed{\text{they form a geometric progression}}$