Răspuns :
( x + 9 ) / 4 = ( x + 12 ) / 5
5 ( x + 9 ) = 4 ( x + 12 ) → produsul mezilor = produsul extremilor
5 x + 45 = 4 x + 48
5 x - 4 x = 48 - 45
x = 3
Verific: ( 3 + 9 ) / 4 = ( 3 + 12 ) / 5 = 12 / 4 = 15 / 5 = 3
( x + 9 ) / 4 = ( x + 12 ) / 5
5 ( x + 9 ) = 4 ( x + 12 ) → produsul mezilor = produsul extremilor
5 x + 45 = 4 x + 48
5 x - 4 x = 48 - 45
x = 3
Verific: ( 3 + 9 ) / 4 = ( 3 + 12 ) / 5 = 12 / 4 = 15 / 5 = 3