Question

$left { {{ab=13} atop {bc=9}} right.$⇒a+b-c=? (a, b, c ∈ N)

Answer 1

{ab = 13,
{bc = 9;
a, b, c ∈ N ⇒ a = 13, b = 1, c = 9
a + b - c = 14 - 9 = 5

Answer 2

А каким образом?

Answer 3

А все ясно