18)

f(x)=x？ -3x？ -9x-5

1) The domain is the real number range r.

2) Deduction: f'(x)=3x? 6x 9

3) The stagnation point satisfies f'(x)=0.

So: 3x? -6x-9=0

x？ -2x-3=0，(x-3)(x+ 1)=0

X=- 1 or x=3.

The pile numbers are (-1, 0) and (3,32).

4) Derive f'(x): f''(x)=6x-6.

So:

When x=- 1, f'' (- 1) =- 12.

When x=3, f'' (3) =12 > 0 is the minimum point, and the minimum value f(3)=-32.