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cum se calculeaza tipul asta de ecuatie??????

calculati 5a-11b+21c
daca 2a+b-3c=15 si a-4b+8c=25


Răspuns :

[tex]2a+b-3c=15\\a-4b+8c=25 |\cdot3\ (Amplificam\ relatia\ cu\ 3)\\\\2a+b-3c=15\\3a-12b+24c=75 | \ (Adunam\ cele\ doua\ relatii\ ) \\\\2a+3a-12b+b+24c-3c=75+15\\5a-11b+21c=90[/tex]
e simplu,defapt
5a-11b+21c=2a+b-3c+3(a-4b+8c)=2a+b-3c+3a-12b+24c=5a-11b+21c
am aflat ca 2a+b-3c+3(a-4b+8c)=5a-11b+21c
2a+b-3c+3(a-4b+8c)=15+3x25=15+75=90