方程x-1=lgx必有一个根的区间是(  ) A.(0.1,0.2) B.(0.2,0.3) C.(0.3,0.4) D.(0.4,0.5)

问题描述:

方程x-1=lgx必有一个根的区间是(  )
A. (0.1,0.2)
B. (0.2,0.3)
C. (0.3,0.4)
D. (0.4,0.5)

令f(x)=x-1-lgx,
则f(0.1)=0.1-1-lg0.1=0.1>0,f(0.2)=0.2-1-lg0.2=0.2-1-(lg2-1)=0.2-lg2,

lg2
0.2
=lg2 
1
0.2
=lg32>lg10=1;
∴lg2>0.2;f(0.2)<0;
同理:(0.3)=0.3-1-lg0.3=0.3-1-(lg3-1)=0.3-lg3<0
f(0.4)<0
∴在区间(0.1,0.2)上必有根,
故选:A.