a-1/2(a+a/2)a-1/2(a-a/2)a-1/2*a^2/2
=a-1/2(3a/2)a-1/2(a/2)a-1/4*a^2
=a-3/4*a^2-1/4*a^2-1/4*a^2
=a-(3/4+1/4+1/4)*a^2
=a-5/4*a^2a^2+(a/2)^2-1/2(a+a/2)a-1/2(a-a/2)a-1/2*a^2/2这个呢？a^2+(a/2)^2-1/2(a+a/2)a-1/2(a-a/2)a-1/2*a^2/2=a^2+a^2/4-3a^2/4-a^2/4-a^2/4=(1+1/4-3/4-1/4-1/4)a^2=0采纳吧