用推导法：
因为d=2n-1(n大于等于1且为正整数)
所以c=d+2,b=d+4,a=d+6
ac-bd
=(d+6)(d+2)-(d+4)d
=d^2+8d+12-d^2-4d
=4d+12
=4(2n-1)+12
=8n+8(n大于等于1且为正整数)
或者
ac=b^2-4
bd=c^2-4
c=d+2=2n+1
b=c+2=2n+3
ac-bd
=b^2-c^2
=(b+c)(b-c)
=2(b+c)
=2[(2n+3)+(2n+1)]
=8n+8(n大于等于1且为正整数)