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VA ROG!!! Calculati 7a+9b+9c+7d,stiind ca a+b+c+d=30 si a+d=10 DAU COROANA CINE STIE!!!!!!!

Răspuns :

a + b + c + d = 30
a + d = 10
b + c + 10 = 30
b + c = 30 - 10 = 20

7a + 9b + 9c + 7d =

= (7a + 7d ) + (9b + 9c) =

= 7×(a+d) + 9 × (b+c) =

= 7 × 10 × 9 × 20 =

= 70 + 180 =

= 250