由换底公式logа(b)=logA(b)／logA(a)
{ 例如,以3为底2的对数log3(2)=log10(2)／log10(3)=lg(2)／lg(3)或者其他底数}
原式=log3(2)×log5(7)／[log9(1/7)×log125(8)]
=﹙lg(2)／lg(3)﹚×﹙lg(7)／lg(5)﹚／[﹙lg(1/7)／lg(9)﹚×﹙lg(8)／lg(125)]
=﹙lg(2)／lg(3)﹚×﹙lg(7)／lg(5)﹚／[﹙-lg(7)／2lg(3)﹚×﹙3lg(2)／3lg(5)]
=-2