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If f(x)= (ax+b)/(cx+d), abcd ≠ 0 and f(f(x))=x for all x in the domain of f, what is the value of a+d?

 Feb 20, 2021
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f(f(x)) = (a(ax + b)/(cx + d) + b)/(c(ax + b)/(cx + d) + d).

Expanding and matching coefficients gives you a + d = 2.

 Feb 20, 2021

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