# $$f(x)=\frac{a^x-a^{-x}}{a^x+a^{-x}}$$

Algebra Level 2

For $f(x)=\frac{a^x-a^{-x}}{a^x+a^{-x}} ( a \neq 1, a>0),$ if $$f(k)=10,$$ what is the value of $$f(2k)?$$

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