设向量 $a_k=(\cos \frac{k\pi }{6},\sin \frac{k\pi }{6}+\cos \frac{k\pi }{6})(k=0,1,2,...,12)$，则 $\underset{k=0}{\overset{11}{\sum }}(a_k·a_{k+1})$的值为.