F(-7) = 3/7*(-7) - l = -3 - i = -5,9; l = -5,9 + 3 = -2,9
f(2 1/3) = f(7/3) = 3/7*7/3 - l = 1 - l = 4/9; l = 1 - 4/9 = 5/9
f(-2,8) = f(-7*0,4) = 3/7*(-7*0,4) - l = -1,2 - l = -2; l = 2 - 1,2 = 0,8
f(1,75) = f(7*0,25) = 3/7*(7*0,25) - l = 3/4 - l = -1/4; l = 3/4 + 1/4 = 1