Question 535345
{{{ f(x)= log((x^2-17x+72)) }}}
" logs are exponents " That's worth repeating.
It's important how you read this equation.
{{{ x^2-17x+72 }}} is what I get when I 
raise {{{10}}} to the exponent {{{ f(x) }}}
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The thing to see here is that there is no way
you can end up with a negative by raising
{{{10}}} to some power like {{{ f(x) }}}
so,  {{{ x^2-17x+72 }}} can't be negative.
But, the log of this can be negative, 
positive or zero
the domain is
- infinity < f(x) < + infinity