c)
(x-4y)^2-(4y+x)^2- dla x= \frac{1}{4} , y= \frac{\sqrt{3}}{4}
(x-4y)^2+ ( x-4y)^2 =2(x-4y)^2
2(\frac{1}{4}-\frac{\sqrt{3}}{4})^2=2*\frac{1}{4^2}*(1- \sqrt{3} )^2=\frac{1}{8}*(1-2\sqrt{3}+3)=\frac{1}{8}*(4-2\sqrt{3})=
=\frac{1}{8}*2 (2-\sqrt{3}) =\frac{1}{4}(2-\sqrt{3})= \frac{1}{2}-\frac{\sqrt{3}}{4}