how can we write 4√2 as a decimal

Answer 1

since √2 is irrational, it cannot be written accurately as a decimal
The best you can do is write it correct to some certain number of decimal places, rounding it off

e.g. correct to 3 decimals:
4√2 = 4(1.41421356.....)
= 5.657 correct to 3 decimals
= 5.66 correct to 2 decimals
= 5.7 correct to one decimal

Answer 2

I think it is 1.41418384747464646465665656575786688695949494393939484848488575 and reapeated.