lg5
=lg(10/2)
=lg10-lg2
=1-0.3010
=0.6989;
lg12
=lg(4*3)
=lg4+lg3
=lg2²+lg3=
2lg2+lg3
=2*0.3010+0.4771
=1.0791;
log(1/2)90
=lg90/lg(1/2) [换底公式]
=lg(3²*10)/lg(1/2)
=(lg3²+lg10)/(lg1-lg2)
=(2lg3+1)/(0-lg2)
=(2*0.4771+1)/(-0.3011)
=-6.4902