设$a,b,c$均为不等于1的正实数, 则下列等式中恒成立的是

${\log }_{a}b•{\log }_{c}b={\log }_{c}a$ ${\log }_{a}b•{\log }_{a}a={\log }_{a}b$ ${\log }_{a}bc={\log }_{a}b•{\log }_{a}c$ ${\log }_a(b+c)={\log }_{a}b+{\log }_{a}c$