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5.   9^1/3  *  81^1/3   =   ( 9 * 81 )^1/3   =   ( 9 * 9 * 9 )^1/3   =   (9³)^1/3   =   9^3/3   =   9¹   =   9

 

6.   \(\frac{8}{\sqrt[7]{x^{15}}}\,=\,\frac{8}{x^{\frac{15}{7}}}\,=\,8x^{-\frac{15}{7}}\)