(2)4 (let t=a-2, then t >；; 0，a+[ 1/(a-2)]= t+2+ 1/t≥2+2 = 4

(3)[2,+∞) (-∞,-2]∪[2,+∞)

(4)x=2 Minimum value 4

Basic inequality: (a+b)/2≥ root sign (ab)

Grasp the seven-character formula: "one positive, two definite and three equal"

A positive means that all the numbers in the question must be positive.

Two, that is, one of a+b and ab must be a constant value.

Three-phase equivalence means that the condition of equal sign is a=b, which is very important in finding the maximum value.

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