b计算x(k)的4点DFT X(p)=∑xp=02,3 k=0 j2x/4 4b.计算x(k)的4点DFT. W e j X p x k W p j k kp = = − = = − =  2 / 4 4 3 0 ( ) ( ) 4 0,1,2,3 