ax^2+bx+c=0
4x^2-c=0
a=4, b=0, c>0
\Delta=b^2-4ac=0-4*4*{-c}=16c
\sqrt\Delta=\sqrt{16c}=4\sqrt{c}
x_1=\frac{-b-\sqrt\Delta}{2a}=\frac{-0-4\sqrt{c}}{2*4}=\frac{-\not4\sqrt{c}}{2*\not4}=-\frac{\sqrt{c}}{2}
x_2=\frac{-b+\sqrt\Delta}{2a}=\frac{-0+4\sqrt{c}}{2*4}=\frac{\not4\sqrt{c}}{2*\not4}=\frac{\sqrt{c}}{2}
x_1*x_2=-\frac{\sqrt{c}}{2}*\frac{\sqrt{c}}{2}=-\frac{c}{4}