Cho a^2 + b^2 + c^2 = 0. Chứng minh rằng A = B = C với A= a^2(a^2+b^2)(a^2+c^2)

\[A = {a^2}\left( {{a^2} + {b^2}} \right)\left( {{a^2} + {c^2}} \right)\]

\( = \left( {{a^4} + {a^2}{b^2}} \right)\left( {{a^2} + {c^2}} \right)\)

\( = {a^6} + {a^4}{c^2} + {a^4}{b^2} + {a^2}{b^2}{c^2}\)

\( = {a^4}\left( {{a^2} + {c^2} + {b^2}} \right) + {a^2}{b^2}{c^2}\)

\( = {a^4}.0 + {a^2}{b^2}{c^2}\)

\( = {a^2}{b^2}{c^2}\).                        (1)

\(B = {b^2}\left( {{b^2} + {c^2}} \right)\left( {{b^2} + {a^2}} \right)\)

\( = \left( {{b^4} + {b^2}{c^2}} \right)\left( {{b^2} + {a^2}} \right)\)

\( = {b^6} + {b^4}{a^2} + {b^4}{c^2} + {a^2}{b^2}{c^2}\)

\( = {b^4}\left( {{b^2} + {a^2} + {c^2}} \right) + {a^2}{b^2}{c^2}\)

\( = {b^4}.0 + {a^2}{b^2}{c^2}\)

\( = {a^2}{b^2}{c^2}\).                         (2)

\(C = {c^2}\left( {{c^2} + {a^2}} \right)\left( {{c^2} + {b^2}} \right)\)

\( = \left( {{c^4} + {c^2}{a^2}} \right)\left( {{c^2} + {b^2}} \right)\)

\( = {c^6} + {c^4}{b^2} + {c^4}{a^2} + {a^2}{b^2}{c^2}\)

\( = {c^4}\left( {{c^2} + {b^2} + {a^2}} \right) + {a^2}{b^2}{c^2}\)

\( = {c^4}.0 + {a^2}{b^2}{c^2}\)

\( = {a^2}{b^2}{c^2}\).                         (3)