( 4 x - 1) / 9 = 8 / 21
21 ( 4 x - 1 ) = 9 · 8 → produsul mezilor = produsul extremilor
84 x - 21 = 72
84 x = 72 + 21
x = 93 / 84 = 31 / 28
b)
( 5 x - 1 ) / 8 = 9 / 11
11 ( 5 x - 1) = 8 × 9
55 x - 11 = 72
55 x = 72 + 11
55 x = 83
x = 83 / 55
c)
( 8 x - 1) / 13 = 5 / 16
16 ( 8 x - 1 ) = 5 × 13
128 x - 16 = 65
128 x = 65 + 16
128 x = 81
x = 81 / 128