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4a +3b +7c =24 7a +9b +c=42 a+b+c=?


Răspuns :

4a+3b+7c=24 (1;)
7a+9b+c=42   (2)        a+b+c=?

din (2) c=42-7a-9b
(1) 4a+3b+294-49a-63b=24
-45a-60b=-270 |:(-3)
15a+20b=90 |:5
3a+4b=18 |·2
6a+8b=36

7a+9b+c=42
6a+8b    =36
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a  + b+c = 6