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daca a+b=20 si b+c=30, calculati 3xa+7xb+4xc

Răspuns :

3a+7b+4c=a+a+a+b+b+b+b+b+b+b+c+c+c+c=3a+3b+4b+4c=3x20+4x30=
60+120=180
a+b=20⇒3a+3b=60
b+c=30⇒4b+4c=120
Adunam: 3a+(4b+3b)+4c=60+120
                3a+7b+4c=180
Sper ca te-am ajutat.