If T is the number of turning points of an iid sequence of length n,since the probability of a turning point at time i is 2/3 ,the variance of T is :(16n-29)/90 .why?  这个题目T不是应该服从二项分布吗b（n-2，2/3）？请各位DX指点。。。。

There is a turning point at time i, 1<i<n,if y(i-1)<y(i)and y(i)>y(i+1) or y(i-1)>y(i)and y(i)<y(i+1) .