3 4 = 8 1, 1998/4 = 499 Yu 2

3^ 1998=

(3^4)^499

*

3^2

Of which (3 4) 499

The unit number is 1,

3 2 digits are: 9, 1*9=9,

So the digit of 3 1998 is 9.

Second,

5 to the power of 1999:

What is the power of five digits?

The power of 1999 of 5 is 5.

Three,

7 4 = 8 1, the unit number is1.

2000/4=500,

7^2000=(7^4)^500

So the unit number of 7 to the 2000th power is 1.

The unit number is 9*5* 1=45, and the unit number is 5.

The unit number is 5.