\magnification=\magstep1

$$
{ {I_A} \over {I_B} } \quad = \quad {\left( { {d_B} \over {d_A} } \right)}^2
$$

$$
d_B \quad = \quad d_A  \thinspace \sqrt{ { { I_A } \over {I_B} }  }
$$


$$
{ {I_A} \over {I_B} } \quad = \quad 10^{ \thinspace 0.4({\rm m}_B - {\rm m}_A) }
$$

$$
d_B \quad = \quad d_A  \thinspace \thinspace 10^{ \thinspace 0.2({\rm m}_B - {\rm m}_A) }
$$



\bye
