You should not understand the third to fourth steps. The method is just to multiply both sides of the third step by 8, and then add a D 2 to both sides at the same time, so that the left side forms a formula that can be simplified by the complete square formula, and then move 8 from left to right, so that the fourth step is like this. It is found that in the fourth step, the left side is greater than or equal to 0, and so is the right side, so it is deduced that D 2 is greater than or equal to 8.

So I got this answer. ...