（a-1/2)^2=a^2-a+1/4 ;（a+1/2)^2=a^2+a+1/4
[（a-1/2)^2+（a+1/2)^2]*[2a^2-1/2]=][2a^2+1/2]]*[2a^2-1/2]=4a^4-1/4