\[\boxed{\text{915\ (915).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ a + b = 6,\ \ ab = 3:\]
\[a^{2} + b^{2} = a^{2} + 2ab + b^{2} - 2ab =\]
\[= (a + b)^{2} - 2ab = 6^{2} - 2 \cdot 3 =\]
\[= 30.\]
\[\textbf{б)}\ c + \frac{1}{c} = 2,5:\]
\[c^{2} + \frac{1}{c^{2}} = c^{2} + 2 + \frac{1}{c^{2}} - 2 =\]
\[= \left( c + \frac{1}{c} \right)^{2} - 2 = {2,5}^{2} - 2 =\]
\[= 4,25.\ \]