【成才之路】-学年高中数学3.4对数课后强化作业北师大版必修1一、选择题1.若log8x=-,则x的值为()A.B.4C.2D.[答案]A[解析]∵log8x=-,∴x=8-=2-2=,故选A.2.当a>0,a≠1时,下列结论正确的是()①若M=N,则logaM=logaN;②若logaM=logaN,则M=N;③若logaM2=logaN2,则M=N;④若M=N,则logaM2=logaN2.A.①②B.②④C.②D.①②③④[答案]C[解析]①M≤0时不对;②正确;③应为M=±N;④M=0时不对.3.lg20+lg50的值为()A.70B.1000C.3D.[答案]C[解析]lg20+lg50=lg1000=3.故选C.4.已知a=log32,那么log38-2log36用a表示是()A.a-2B.5a-2C.3a-(1+a)2D.3a-a2-1[答案]A[解析]log38-2log36=log323-2(log32+log33)=3log32-2(log32+1)=3a-2(a+1)=a-2.故选A.5.若log2[log3(log4x)]=log3[log4(log2y)]=log4[log2(log3z)]=0,则x+y+z=()A.50B.58C.89D.111[答案]C[解析]∵log2[log3(log4x)]=0,∴log3(log4x)=1,∴log4x=3,∴x=43=64,同理y=16,z=9,∴x+y+z=89,故选C.6.方程log3(x-1)=log9(x+5)的解为()A.x=-1B.x=-1或x=4C.x=4D.x=-1且x=4[答案]C[解析]一定要注意对数的真数大于零,即,解得x=4,选C.二、填空题7.求值:(1)810.5log35=________;(2)5log5100-3=________;(3)27+log32=________.[答案](1)25(2)(3)72[解析](1)810.5log35=(34)0.5log35=32log35=(3log35)2=52=25.(2)5log5100-3===.8.设2a=5b=m,且+=2,则m=________.[答案][解析]∵a=log2m,b=log5m,∴+=+=logm2+logm5=logm10=2,∴m2=10.又∵m>0,∴m=.三、解答题9.计算下列各式的值:(1)log2+log212-log242;(2)lg52+lg8+lg5·lg20+(lg2)2.[解析](1)原式=log2+log212-log2=log2(··12)=log2(··12)=log2=log22-=-.(2)原式=2lg5+2lg2+lg5·(1+lg2)+(lg2)2=2(lg5+lg2)+lg5+lg2(lg5+lg2)=2+lg5+lg2=2+1=3.一、选择题1.等于()A.lg3B.-lg3C.D.-[答案]C=+=+==.2.如果f(10x)=x,则f(3)等于()A.log310B.lg3C.103D.310[答案]B[解析]令10x=3,∴x=lg3.故选B.二、填空题3.(1)已知a=(a>0),则a=________.(2)已知m>0,且10x=lg(10m)+lg,则x=________.[答案](1)3(2)0[解析](1)由a=(a>0),得a=()=()3,所以a=()3=3.(2)10x=lg(10m·)=lg10=1.所以x=0.4.若正数m,满足10m-1<2512<10m,则m=__________.(lg2≈0.3010)[答案]155[解析]∵10m-1<2512<10m,∴m-1<512lg20,y>0,x-2y>0得x>2y>0.∴x=y应舍去,故=4.∴log=log4=4.