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calculati 5a-11b+21c, stiind ca 2a+b-3c= 15 si a - 4b+8c = 25,unde a,b,c apartine er R 

Răspuns :

5a-11b+21c=2a+b-3c+3*(a-4b+8c)=15+3*25=15+75=90