29. A regular triangular prism with a base side of $a$ is inscribed in a sphere of radius $R$. Find the area of the section of the prism by a plane passing through the center of the sphere and a side of the base of the prism.
29. $\frac{2 a}{3} \sqrt{4 R^{2}-a^{2}}. 

.

Translate the above text into English, keeping the original text's line breaks and format, and output the translation result directly.

29. $\frac{2 a}{3} \sqrt{4 R^{2}-a^{2}}.