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Simplify, $(b^{1/2} - 4b^{-1/2})(b^{1/2} + 4b^{-1/2}) $

Is answer $-15b$?

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    $(b^{1/2} - 4b^{-1/2})(b^{1/2} + 4b^{-1/2}) = \dfrac{(b-4)(b+4)}{b}$ – 2017-01-09

2 Answers 2

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it can be simplified after the binomial formula $$(a-b)(a+b)=a^2-b^2$$ after this formula we get $$b-16b^{-1}$$

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    Thanks! I went through again and got this to be my answer too. Unsure where I went wrong the first time. – 2017-01-09
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Put $a=b^{\frac{1}{2}}$ then the expression becomes

$$(a-\frac{4}{a})(a+\frac{4}{a})$$

$$=a^2-\frac{16}{a^2}$$

$$=b-\frac{16}{b}.$$