A median is a line segment that connects a vertex of a triangle to the midpoint of the opposite side. I need to find the length of each median and then square them and add them up. To find the length of a median, I can use the formula $m = \sqrt{\frac{2a^2+2b^2-c^2}{4}},$ where $m$ is the median, $a$ and $b$ are the sides that the median splits, and $c$ is the side that the median meets at a vertex. I can label the sides of the triangle as $a = 10,$ $b = 10,$ and $c = 12.$ Then, I can use the formula to find the length of each median. For the median that splits the side of length $12,$ I get $m = \sqrt{\frac{2(10^2)+2(10^2)-12^2}{4}} = \sqrt{\frac{200+200-144}{4}} = \sqrt{\frac{256}{4}} = \sqrt{64} = 8.$ For the median that splits one of the sides of length $10,$ I get $m = \sqrt{\frac{2(10^2)+2(12^2)-10^2}{4}} = \sqrt{\frac{200+288-100}{4}} = \sqrt{\frac{388}{4}} = \sqrt{97}.$ For the other median that splits the other side of length $10,$ I get the same value as before, since the triangle is isosceles and the two medians are symmetric. So, the lengths of the medians are $8,$ $\sqrt{97},$ and $\sqrt{97}.$ To find the sum of the squares of the lengths of the medians, I need to calculate $8^2 + (\sqrt{97})^2 + (\sqrt{97})^2.$ This simplifies to $64 + 97 + 97 = 258.$ 
What is the sum of the squares of the lengths of the $\textbf{medians}$ of a triangle whose side lengths are $10,$ $10,$ and $12$?