According to Vieta's Formulas, if 
are solutions of a given quadratic equation:
Then:
is the completely factored form of 
.
If choose
, then:
So, according to Vieta's formula, we can get:
But:
![d^4-8d^2+16=(d^2-4)^2=[(d+2)(d-2)]^2=(d+2)^2(d-2)^2](https://tex.z-dn.net/?f=d%5E4-8d%5E2%2B16%3D%28d%5E2-4%29%5E2%3D%5B%28d%2B2%29%28d-2%29%5D%5E2%3D%28d%2B2%29%5E2%28d-2%29%5E2)
Then:
If choose
So, according to Vieta's formula, we can get:
But