18. a) y = x²-6x+7; б) y = -12/x; в) y = √x +5.

18. a) $$y = x^2 - 6x + 7$$

б) $$y = -\frac{12}{x}$$

в) $$y = \sqrt{x} + 5$$

Ответ: $$y = x^2 - 6x + 7$$, $$y = -\frac{12}{x}$$, $$y = \sqrt{x} + 5$$