Answer
$(pc)^2 = K^2+2KE_0$
Work Step by Step
We can find an expression for $(pc)^2$:
$E^2 = E_0^2+(pc)^2$
$(K+E_0)^2 = E_0^2+(pc)^2$
$K^2+2KE_0+E_0^2 = E_0^2+(pc)^2$
$(pc)^2 = K^2+2KE_0$
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