【(a-二分之一)的二次方-(a+二分之一)的二次方】(a+3)
=[(a-1/2)-(a+1/2)][(a-1/2)+(a+1/2)](a+3)
=-1*2a*(a+3)
=-2a*(a+3)
=-2*(-2)*(-2+3)
=4