Question 1118792
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f(x) = x^2 - bx + 9.


f(x) has one rational root if and only if the discriminant of this quadratic equation is zero


d = (-b)^2 - 4*1*9 = 0,   or   b^2 - 36 = 0,


which implies  b = +/-6.


Since the problem requires  b > 0,  there is only one solution  b = 6.
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