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1) Cho [tex]\frac{ay - bx}{c} = \frac{cx - az}{b} = \frac{bz - cy}{a}[/tex] . Chứng minh [tex](ax+by+cz)^{2} = (a^{2}+ b^{2} + c^{2} )(x^{2}+ y^{2} + z^{2} )[/tex]
2)Cho [tex]\frac{x^{2}-yz}{a} = \frac{y^{2}-xz}{b} = \frac{z^{2}-xy}{c}[/tex]. Chứng minh [tex]\frac{a^{2}-bc}{x} = \frac{b^{2}-ac}{y} = \frac{c^{2}-ab}{z}[/tex]
2)Cho [tex]\frac{x^{2}-yz}{a} = \frac{y^{2}-xz}{b} = \frac{z^{2}-xy}{c}[/tex]. Chứng minh [tex]\frac{a^{2}-bc}{x} = \frac{b^{2}-ac}{y} = \frac{c^{2}-ab}{z}[/tex]