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daca se stie a+b=21,calculati 7a+7b

Răspuns :

a+b=21
a=21-b
7a+7b= 7(21-b)+7b
141-7b+7b=141   deci 7a+7b = 141

a+b=21
7a+7b=(a+b)x7=>21x7=147