So, the number of nuclei, 119 if we were to sit and count these as well, is 119. So, we'll multiply that by just pi, r squared, to get that cross-section, and divide all of that by 1 . 39 meters squared.

如果你们数的话，原子核的数是，我们用它乘以πr的平方，得到横截面积，除以1。39平方米。

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