2. Two-digit number: there is a 7 8+9 bit, and there are two 7 1 bits, *** 18 bits;

3. Three digits: a seven-hundred-yuan 9*9=8 1, a ten-yuan 8*9=72, a unit 8*9=72, two sevens 9+8+9=26, and three sevens 1, * *.

4, four digits ...

You can do it in the above way.

You can also use the following methods (indirect method, minus the method without 7):

1, one digit:10-9 = 9-8 =1;

2. Two digits: 9× 10-8×9.

3. Three digits: 9× 10× 10-8×9×9.

4. Four digits: 9×10×10×10-8× 9× 9.

5. Five digits: 9×10×10×10×10-8× 9× 9.

……

9. Nine digits: 9× 10 8-8× 9 8

10, decimal places: when the highest digit is 1,109-99;

When the highest digit is 2, there is 1 in millions, and 8× 10 6-7×9 6 in millions is not 8 (0-7).

Add up the above results:

9( 1+ 10+ 10^2+……+ 10^8)-8( 1+9+9^2+……9^8)+ 10^9-9^9+× 10^6-7×9^6+ 1

In brackets are geometric series, which can be calculated by the summation formula of equal ratio series.

The following should be easy to calculate.

There is no need to make such a big number on this topic, just make it smaller, and more importantly, master the method:

The former is the direct method and the latter is the indirect method.