18. lna/lnx=2→lna=2lnx
lnb/lnx=3→lnb=3lnx
lnc/lnx=6→lnc=6lnx
∴lna+lnb+lnc=ln(abc)=11lnx
log(abc)X=lnx/ln(abc)=1/11
19.=(½lg3³)+lg2³-½ln10³)/lg1.2
=(1.5lg3+1.5lg4-1.5lg10)/lg1.2
=1.5lg(3·4/10)/lg1.2
=1.5lg1.2/lg1.2
=1.5
解，logx(a)=lga/Igx=2则lga=2Igx
lgb=3lgx，Igx=6lgx
则Iogabc(x)=lgx/Igabc=Ig/(lga+lgb+lgC)
=1/10
19，lg√27+1g8-lg√1000
=lg(3^3/2)+lg4^3/2-lg(10^(3/2)
=3/2(Ig3+1g4-1)
=3/2(1g3x4÷10)
=3/2lg1.2
则原式=3/2。
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