Answer
$r=5$
Work Step by Step
$a^{2}+c^{2}=d^{2}$
$r^{2}+12^{2}=(8+r)^{2}$
$r^{2}+144=(8+r)^{2}$
$r^{2}+144=r^{2}+16r+64$
$144=16r+64$
$144=16r+64$
$80=16r$
$r=5$
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