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7×a+9×b+2×c daca a+b=43 si b+c=15

Răspuns :

a+b=43 |×7
7×a + 7×b = 301

b+c=15 |×2
2×b + 2×c = 30

adunam cele doua relatii rezultate:
7×a + 7×b + 2×b + 2×c =
= 7×a + 9×b + 2×c = 301 + 30 = 331