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Let \(f\) and \(g\) be two functions defined by \(f(x) = |x + |x||\) and \(g(x) = \frac{1}{x}\) for \(x \neq 0\). If \(f(a) + g(f(a)) = \frac{13}{6}\) for some real \(a\), then the maximum possible…

Question and options

Let ff and gg be two functions defined by f(x)=∣x+∣x∣∣f(x) = |x + |x|| and g(x)=1xg(x) = \frac{1}{x} for x≠0x \neq 0. If f(a)+g(f(a))=136f(a) + g(f(a)) = \frac{13}{6} for some real aa, then the maximum possible value of f(g(a))f(g(a)) is:
  1. 6
  2. 9
  3. 12
  4. 3

Answer and explanation

Correct answer: A - 6

The maximum is 6. We need a>0a>0 so that f(a)=2af(a)=2a is nonzero, and with t=2at=2a the equation t+1/t=13/6t+1/t=13/6 gives t=2/3t=2/3 or t=3/2t=3/2. Then f(g(a))=2/a=4/tf(g(a))=2/a=4/t, whose maximum comes from t=2/3t=2/3. The value 3 is the reciprocal of the smaller aa before applying the outer function.

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