30. The sum of the two diagonals of a rhombus is $l$, and the area is $S$. Then its side length is ( ).
(A) $\frac{1}{2} \sqrt{4 S-l^{2}}$
(B) $\frac{1}{2} \sqrt{4 S+l^{2}}$
(C) $\frac{1}{2} \sqrt{l^{2}-4 S}$
(D) $\frac{1}{2} \sqrt{l^{2}+4 S}$
30.C.

Let the lengths of the diagonals of the rhombus be $a$ and $b$, then $a+b=l$, $\frac{1}{2} a b=S$.

Therefore, the side length of the rhombus is $\frac{1}{2} \sqrt{a^{2}+b^{2}}=\frac{1}{2} \sqrt{l^{2}-4 S}$. Note: The reference answer is provided by Song Qiang, the editor of our journal.
（Provided by Nü Dao）