6△(t) (t) has unit area x(k△) (k△)06△(t-k△)△ k△个 (k+1)△ ()=∑(k△A△(t-kA)△ k=- l limit as△→0 x(T)6(t-T) The Sifting Property of the Unit Impulsehas unit area The Sifting Property of the Unit Impulse