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a+b=5 b+c=7 2a+5b+3c este??

Răspuns :

2a+2b+3b+3c = 
2(a+b) + 3(b+c) = 
2*5+3*7 = 
10+21=
31
a+b=5 I *2
b+c=7 I *3

2a+2b=10
3b+3c=21(+)

2a+2b+3b+3c=10+21
2a+5b+3c=31