а) f(x+dx) - f(x) = -(x+dx)^2 + 8(x+dx) + x^2 - 8x = -2xdx - dx^2 - 8dx < {x >= 4, dx > 0} < 0

б) g(x+dx) - g(x) = -2/(x+dx-3) + 4 + 2/(x-3) - 4 = 2[(x+dx-3) - (x-3)]/[(x+dx-3)(x-3)] = 2dx/[(x-3)(x+dx-3)] > {x > 3, dx > 0} > 0