
 begin{align} x(t) &=cos(2pi a t)cos(2pi b t)cos(2pi c t) notag\ &={1}/{2}cos(2pi c t)left{% cos[2pi (a+b) t]+cos[2pi (a-b) t]...
 图中的[14]为何不是[1],[4]为何不是[2],[11]为何不是[3],
 图中的[14]为何不是[1],[4]为何不是[2],[11]为何不是[3],


