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a fost răspuns

Calculati 5a-11b+21c , stiind ca 2a+b-3c=15 si 1-4b+8c=25 unde a,b,c apartine R

Ajutor!!

Răspuns :

5a-11b+21c=2a+b-3c+3(a-4b+8c)
5a-11b+21c=2a+b-3c+3a-12b-24c
5a-11b+21c=5a-11b+21c
apoi inlocuiesti
15+3*25
15+75=90