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[tex] {3}^{ - 3} + {9}^{ - 2} \\ = \frac{1}{ {3}^{3} } + \frac{1}{ {9}^{2} } \\ = \frac{1}{27} + \frac{1}{81} \\ = \frac{3}{81} + \frac{1}{81} \\ = \frac{4}{81} = \frac{{2}^{2} }{ {3}^{4} } [/tex]

[tex]3 { }^{ - 3} + 9 {}^{ - 2} = \frac{1}{ {3}^{3} } + \frac{1}{ {9}^{2} } = \frac{1}{27} + \frac{1}{81} = \frac{3 + 1}{81} = \frac{4}{81} [/tex]