Rút gọn biểu thức: \( C = 9{x^2} - 2xy + \frac{1}{9}{y^2} - 2\left( {3x - \frac{1}{3}y} \right)\left( {3x + \frac{1}{3}y} \right) + {\left( {3x + \frac{1}{3}y} \right)^2}\)
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Lời giải:
Báo sai\(\begin{array}{l} C = 9{x^2} - 2xy + \frac{1}{9}{y^2} - 2\left( {3x - \frac{1}{3}y} \right)\left( {3x + \frac{1}{3}y} \right) + {\left( {3x + \frac{1}{3}y} \right)^2}\\ \Leftrightarrow C = {(3x)^2} - 2.3x.\frac{1}{3}y + {\left( {\frac{1}{3}y} \right)^2} - 2\left( {3x + \frac{1}{3}y} \right)\left( {3x - \frac{1}{3}y} \right) + {\left( {3x + \frac{1}{3}y} \right)^2}\\ \Leftrightarrow C = {\left( {3x - \frac{1}{3}y} \right)^2} - 2\left( {3x + \frac{1}{3}y} \right)\left( {3x - \frac{1}{3}y} \right) + {\left( {3x + \frac{1}{3}y} \right)^2}\\ \Leftrightarrow C = {\left( {\left( {3x - \frac{1}{3}y} \right) - \left( {3x + \frac{1}{3}y} \right)} \right)^2}\\ \Leftrightarrow C = {\left( {3x - \frac{1}{3}y - 3x - \frac{1}{3}y} \right)^2}\\ \Rightarrow C = {\left( { - \frac{2}{3}y} \right)^2} = \frac{4}{9}{y^2} \end{array}\)