# A crystal has a coefficient of linear expansion $\;13\times 10^{-7}/^{0}C\;$ , $\;12 \times 10^{-7}/^{0}C\;$ and $\;10 \times 10^{-7}/^{0}C\;$ in three mutually perpendicular directions respectively . Then the coefficient of cubical expansion of the crystal is

$(a)\;12.5\times 10^{-7}/^{0}C\qquad(b)\;25\times 10^{-7}/^{0}C\qquad(c)\;35\times 10^{-7}/^{0}C\qquad(d)\;17.5\times 10^{-7}/^{0}C$

Answer : $\;35\times 10^{-7}/^{0}C$
Explanation :
$\gamma = \alpha_{1} + \alpha_{2} + \alpha_{3}$
$=35 \times 10^{-7} /^{0}C$