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daca a +b =5 si b+c=7,atunci 2a+5b+3c este.......
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Răspuns :

2a+5b+3c=2a+2b+3b+3c=2(a+b)+3(b+c)=2x5+3x7=10+21=31
a+b=5
a=5-b
b+c=7
c=7-b
si inlocuim
2(5-b) +5b + 3(7-b) =10-2b +5b + 21 - 3b = 10+ 21 -2b + 5b -3b = 31 + 3b -3b = 31