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a) Stiind ca a•b=3/7si c=14/15,calculati a•(b•c).
b)Stiind ca a•b=21/17 si c=51/14,calculati (a•b)•c.
c)Stiind ca a•b=42/5 si c•d=35/77,calculati a•(b•c)•d.