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Math·Advanced Math·Nonlinear functions

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A model estimates that the initial number of photons in an X-ray beam is $700$ when the beam reaches the surface of a certain material. The model also estimates that when the beam passes through this material, the number of photons in the beam decreases by $50\%$ for each $9$ millimeters of the material the beam passes through. Which equation represents this model, where $I$ is the estimated number of photons in the beam when the beam reaches a point $x$ millimeters from the surface?

A

$I=700(0.5{)}^{\frac{x}{9}}$

B

$I=700(0.5{)}^x$

C

$I=700(2{)}^{\frac{x}{9}}$

D

$I=700(9{)}^{\frac{x}{2}}$