The sun comes up in the east and goes down in the west. When the sun rises， It is morning. When the sun sets， it is evening. When the sun is shining， it is day. Morning is the time between sunrise and twelve o'clock， or between sunrise and lunch. At twelve o'clock， the sun is the highest in the sky over our heads. The sun is overhead at noon. The sun shines during（在……期间）the day. The moon and the stars shine during the night. When the sun rises， it is light. It's light during the day. During the night， if the moon is not shining， it is dark. What are the days and nights like in summer? In summer the days are longer and the nights are shorter. What about in winter? In winter， the days are shorter and the nights are longer.

$△ABC$中，内角$A,B,C$所对的边分别为$a,b,c$已知$△ABC$的面积为$3\sqrt{15},b-c=2,\cos A=-\frac{1}{4}$，问：（1）求 a 和 sin C 的值（2）求 cos 2 A + π 6 的值