8. $R=10^{21}-1$
$R^2=10^{42}-2\cdot 10^{21}+1$
$\quad = 10^{21}(10^{21}-2)+1$
$10^{21}-2=\underbrace{99\cdots 9}_{20}8 $
$10^{21}(10^{21}-2)=\underbrace{99\cdots 9}_{20}8\underbrace{00\cdots 0}_{21} $
$10^{21}(10^{21}-2)+1=\underbrace{99\cdots 9}_{20}8\underbrace{00\cdots 0}_{20}1$
ดังนั้น $R^2=\underbrace{99\cdots 9}_{20}8\underbrace{00\cdots 0}_{20}1 $
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