F(x) is the subtraction function on R. If f (4x 2-8x-8) < f (k) 4x 2-8x-8, the minimum value is 4-8-8=- 14 f(x) is the subtraction function on R.

K & lt- 14

The minimum value on 4x 2-8x-8(x≥ 10) is 400-80-8=3 12.

k & gt0 10K+ 12 & lt； 3 12 10k & lt； 300k & lt； 30 0 & ltk & lt30