Giải thích các bước giải: $\dfrac{x}{a}+\dfrac{y}{b}+\dfrac{z}{c}=1\\ \rightarrow (\dfrac{x}{a}+\dfrac{y}{b}+\dfrac{z}{c})^{2}=1\\ \rightarrow (\dfrac{x^{2}}{a^{2}}+\dfrac{y^{2}}{b^{2}}+\dfrac{z^{2}}{c^{2}})+2(\dfrac{xy}{ab}+\dfrac{yz}{bc}+\dfrac{xz}{ca})=1\\ \rightarrow (\dfrac{x^{2}}{a^{2}}+\dfrac{y^{2}}{b^{2}}+\dfrac{z^{2}}{c^{2}})+2\dfrac{xyz}{abc}(\dfrac{a}{x}+\dfrac{b}{y}+\dfrac{c}{z})=1\\ \rightarrow (\dfrac{x^{2}}{a^{2}}+\dfrac{y^{2}}{b^{2}}+\dfrac{z^{2}}{c^{2}})+2\dfrac{xyz}{abc}.0=1\\ \rightarrow \dfrac{x^{2}}{a^{2}}+\dfrac{y^{2}}{b^{2}}+\dfrac{z^{2}}{c^{2}}=1\\$
