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Another piece of nonsense. @rschwieb: This time I wrote a comment.

D1, D2, D3.

D4, D5, D6.

D7, D8, D9.

D10, D11, D12.

C1, C2, C3.

C4, C5, C6.

C7, C8, C9.

@user21820 C4 is open for deletion.

For closure:

3754175

3739948

For deletion:

3749643

3747882

3740884

@user21820 Note that the OP is plagiarising Michael Rozenberg (known as arqady on AoPS)'s solution as his own effort. (The wording is exactly the same e.g. "Thus, it remains to prove..."). It is interesting the victim is aware of it yet he took no action.

@YuiToCheng Well, as far as I know, some users just want to farm rep, and don't care about ethics.

@Saad Both closed now.

@Saad Although the initial version had no effort, the asker did add his attempt before all the answers were posted. Although that attempt was a bit small, I suppose it's acceptable for a new user.

@Saad All three deleted now.

@user21820 All 12 deleted now.

reopen: This question about a differential inequality has been fixed and (I think) sufficient context and solution attempt added. (Disclosure: I have prepared an answer.)

@MartinR Reopened.

@MattSamuel Thanks.

C1. C2

C3