The equation $x^5-2x^4-1=0$ has five complex roots $r_1,r_2,r_3,r_4,r_5$.  Find the value of \[\dfrac1{r_1^8}+\dfrac1{r_2^8}+\dfrac1{r_3^8}+\dfrac1{r_4^8}+\dfrac1{r_5^8}.\]

Let's think step by step and output the final answer within \boxed{}.
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