The vertical line segment OH and h with o as AD are vertical feet,

According to the vertical diameter theorem, OH bisects BC and AD vertically.

Let BC=2m, then BH = ch = m.

Because OA=7 and OC=5, there are:

OC？ -CH？ =OA？ Huh?

5? -m？ =7? -(3+m)？

The solution is m=2.5.

So BC=2m=5cm.

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