a,b互为负倒数
ab=-1
a,c互为相反数
a+c=0
是不是|d|=2?
d=2或-2,d^2=4
d^2-d*[(a+c+ab)/2]^2
=4-(±2)*[(-1+0)/2]^2
=4-(±2)*(1/4)
=4-(±1/2)
所以d^2-d*[(a+c+ab)/2]^2=4+1/2=9/2
或d^2-d*[(a+c+ab)/2]^2=4-1/2=7/2