x2−2y2=18x^{2}-2 y^{2}=18x2−2y2=18
2x2−y2=182 x^{2}-y^{2}=182x2−y2=18
2y2−x2=182 y^{2}-x^{2}=182y2−x2=18
y2−2x2=18y^{2}-2 x^{2}=18y2−2x2=18