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(x2+2y2+2xy)⋅(x2+2y2−2xy)=(x2+2y2)2−4x2y2=x4+4x2y2+4y4−4x2y2=x4+4y4{\displaystyle \left(x^{2}+2y^{2}+2xy\right)\cdot \left(x^{2}+2y^{2}-2xy\right)=\left(x^{2}+2y^{2}\right)^{2}-4x^{2}y^{2}=x^{4}+4x^{2}y^{2}+4y^{4}-4x^{2}y^{2}=x^{4}+4y^{4}}