$f\left(x\right)=16\left(x-0,25\right)\left(x+0,25\right)=16\left(x-\frac{1}{4}\right)\left(x+\frac{1}{4}\right)\stackrel{\left(a-b\right)\left(a+b\right)=a^2-b^2}{=}16\left(x^2-\frac{1}{16}\right)=16x^2-1$