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a) Daca a•b=3 supra 4 si c= 8 supra 9 , calculati a•(b•c); b) Daca a•b=12 supra 13 si c= 26 supra 15, calculati (a•b)•c; c) Daca a= 72 supra 25 si b•c= 35 supra 18, calculati (a•b)•c.