log以56为底的15=log(56)[15],对数函数换底公式log(a)[b]={log(p)[b]}/{log(p)[a]},log(56)[15]={log(15)[15]}/{log(15)[56]}=1/{log(15)[7×8]}=1/{log(15)[7]+log(15)[8]}=1/(a+b),log以3为底的7=a,则log以7为底的...