20.(-∞， 12 1.0，22.a 1=，a2=，an=

20. Solution: Set.

Orders, solutions.

Time, that is, it is an increasing function in the interval;

If, that is, it is a decreasing function in the interval.

The minimum is also the minimum.

If it monotonically increases in the interval at this time,

Therefore, its minimum value is applicable to everyone.

Then, if the interval monotonously decreases at this time,

Therefore, its maximum value is, there is, that is.

So, if it holds true for all, the range of real numbers is.