Try another version of this question Factor `81x^-4-16` `(` `x^-2+` `)(` `x^-2-` `)` `81x^-4-16 = ( 9 x^-2+ 4 )( 9 x^-2- 4 )`
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`(` `x^-2+` `)(` `x^-2-` `)`