IPC as a sub-calculus of PC, and formulas not derivable in IPC

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Problem II.3.42 from Lavrov I.A., Maksimova L.L. "Задачи по теории множеств, математической логике и теории алгоритмов" [LaMa02]More problems in Chapter II.3 "Propositional Calculi"

Prove that:

?
(a)

every formula derivable in IPC is derivable in PC;

(b)

(¬¬P⊃P)(\neg \neg P \supset P), (P∨¬P)(P \vee \neg P) are not derivable in IPC.

References

[LaMa02] Lavrov I.A., Maksimova L.L. Задачи по теории множеств, математической логике и теории алгоритмов. 5-е изд., Физматлит, 2002.