( x + 6 ) / ( x + 9 ) = 13 / 100
100 ( x + 6 ) = 13 ( x + 9 ) → produsul mezilor = produsul extremilor
100 x + 600 = 13 x + 117
100 x - 13 x = 117 - 600
87 x = - 483
x = - 483 / 87
x ≅ - 5,55
