If f (x) = x + (A / x-1) has a minimum value of 4 when x is greater than or equal to 3, then a =? Needs process, good + 100 points

f(x)=(x-1)+(a/x-1)+1
When A0, the function decreases at (0,1 + radical a) and increases at (1 + radical a, positive infinity)
So when 1 + root a

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