2是平方不
(a-2)^2*(a+2)^2=((a-2)(a+2))^2=(a^2-4)^2=a^4+16-8a^2
(a-2)^2*(a+2)^2=[(a-2)(a+2)]^2=(a^2-2^2)^2=a^4-8a^2+16
(a-2)2*(a+2)2=4(a-2)*(a+2)=4(a2_4)=4a2_16
