Answer
$r=6$
Work Step by Step
$a^{2}+c^{2}=d^{2}$
$r^{2}+8^{2}=(4+r)^{2}$
$r^{2}+64=(4+r)^{2}$
$r^{2}+64=16+8r+r^{2}$
$r^{2}+48=8r+r^{2}$
$48=8r$
$r=6$
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