【优化指导】2015-2016高中数学2.2.1第2课时对数的运算学业达标测试新人教A版必修11.若a>0,a≠1,x>0,y>0,x>y,下列式子中正确的个数是()①logax·logay=loga(x+y);②logax-logay=loga(x-y);③loga=logax÷logay;④loga(xy)=logax·logay.A.0B.1C.2D.3解析:根据对数运算性质知4个式子均不正确,③应为loga=logax-logay,④应为loga(xy)=logax+logay.答案:A2.式子的值为()A.B.C.2D.3解析:===.答案:B3.lg8+3lg5的值为()A.-3B.-1C.1D.3解析:lg8+3lg5=lg8+lg53=lg1000=3.答案:D4.若5lgx=25,则x的值为______.解析:∵5lgx=52,∴lgx=2,∴x=102=100.答案:1005.若lga与lgb互为相反数,则a与b的关系是__________________.解析:∵lga+lgb=0,∴lgab=0,∴ab=1.又a>0,b>0,∴ab=1且a>0,b>0.答案:ab=1且a>0,b>06.设log1227=a,求证:log616=.证明:a=log1227==,∴log32=-,log616=4log62=4===.1