第2课时对数的运算(建议用时:40分钟)基础篇一、选择题1.=( )A. B.2C.D.【答案】B [原式=log39=log332=2log33=2.]2.已知a=log32,则log38-2log36=( )A.a-2B.5a-2C.3a-(1+a)2D.3a-a2-1【答案】A [log38-2log36=3log32-2(log32+log33)=3a-2(a+1)=a-2.]3.若lgx-lgy=a,则lg3-lg3等于( )A.3aB.aC.aD.【答案】A [∵lgx-lgy=a,∴lg3-lg3=3lg-3lg=3lgx-3lgy=3a.]4.若a>0,且a≠1,x∈R,y∈R,且xy>0,则下列各式不恒成立的是( )①logax2=2logax;②logax2=2loga|x|;③loga(xy)=logax+logay;④loga(xy)=loga|x|+loga|y|.A.②④ B.①③C.①④D.②③【答案】B [∵xy>0,∴①中,若x0,所以>2,所以=4.]3.=________.【答案】1 [=====1.]
4.若lga,lgb是方程2x2-4x+1=0的两个实根,则ab的值等于________.【答案】100 [∵lga,lgb是方程2x2-4x+1=0的两个实根,∴lga+lgb=-=2,∴ab=100.]5.已知x,y,z为正数,3x=4y=6z,且2x=py.(1)求p;(2)求证-=.【答案】(1)设3x=4y=6z=k(显然k>0,且k≠1),则x=log3k,y=log4k,z=log6k.由2x=py,得2log3k=plog4k=p·.∵log3k≠0,∴p=2log34.(2)证明:-=-=logk6-logk3=logk2,又=logk4=logk2,∴-=.