[tex](x+y) ^{2} = x^{2} +2xy+ y^{2} \ \textless \ =\ \textgreater \ 25=15+2xy=\ \textgreater \ 2xy=10=\ \textgreater \ xy=5. \\ (x-y) ^{2} = x^{2} -2xy+y ^{2} \ \textless \ =\ \textgreater \ (x-y) ^{2} =15-10=5=\ \textgreater \ x-y= \sqrt{5} \\ sau~x-y=- \sqrt{5} . \\ \\ A=(x-y)+(x-y) ^{2} +(x-y) ^{3} = \\ = \sqrt{5}+ (\sqrt{5}) ^{2} + (\sqrt{5})^{3}= \\ = \sqrt{5}+5+5 \sqrt{5}= \\ =\boxed{5+ 6\sqrt{5}}. \\ sau \\ A= -\sqrt{5}+ (-\sqrt{5}) ^{2}+ (- \sqrt{5} )^{3}= \\ = -\sqrt{5}+5- 5\sqrt{5}= \\ =\boxed{5-6 \sqrt{5}} . [/tex]
