Question 1111709
(c,d) is a point on the graph so  {{{ d = log(b, (c)) }}}   —> {{{ b^(d) = c }}} 

If you take the reciprocal of the base, b becomes 1/b and the exponent must be negated to obtain c:
 {{{ (1/b)^(-d) = c  }}}  equivalent to  {{{ log(1/b,(c)) = -d }}} 

thus {{{ highlight( matrix(1,3, " ", " ( c,-d)", " ") )  }}}  must be a point on the graph

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Example/sanity check: 
                 {{{ log(10,(100)) = 2 }}}  because {{{ 10^2 = 100 }}}
                 {{{ log(1/10, (100)) = -2 }}} because {{{ (1/10)^(-2) = (10^(-1))^(-2) = 10^((-1)(-2)) = 10^2 =  100 }}}