(a-1/2)的平方乘以（a+1/2)的平方=(a的平方-1/4)(a的平方-1/4)乘以(a的平方+1/4)=(a的4次方-1/16)
(a+1/2)^2 * (a-1/2)^2 * (a^2+1/4)^2=(a^2-1/4)^2 * (a^2+1/4)^2= (a^4-1/16)^2= a^8 + a^4/8 + 1/256
