Strangely, the incorrect answer keeps getting upvotes even after the gap has been pointed out in a comment. I suppose some voters don't bother to read comments.

[ SmokeDetector | MS ] Korean character in title, mostly non-latin body, mostly non-latin title (268): 많은 OECD 국가들은 성년을 18세로 정하였기 때문에 18세 투표가 당연시된 것이며, ✏️ by Seanmath on math.SE

The incorrect answer Bill pointed out has a nice idea, so I sort of understand why it is popular. I even posted a fix to it. I'm a bit ashamed to admit that the simplest argument only occurred to me when editing a kludgier fix. Comes with the age, I think :-). Bill's fix is, of course, good also (and it cannot hurt to remind the readers about Vieta relations).

@MartinSleziak The question 2659982 (Pythagoras and law of cosines) now has an entry in the reopen request thread.

Delete: DA, DB, DC, DD, DE, DF, DG, DH

D1, D2, D3.

D4, D5, D6.

D7, D8, D9.

D10, D11, D12.

D13, D14, D15.

C1, C2, C3.

C4, C5, C6.

C7, C8, C9.

Delete: DI, DJ, DK, DL, DM, DN, DO, DP