log9(9a+b)=log3√ （ab),0.5log3(9a+b)=log3√ （ab),log3√ (9a+b)=log3√ （ab),9a+b=ab,b=9a/(a-1),4a+b=4a+9a/(a-1)= (4a+5a)/(a-1)=[4(a-2a+1)+13(a-1)+9]/(a-1)=4(a-1)+13+9/(a-1)>=13+2√ [4(a-1)*9/(a-1)]=...