理數is what

rational number is a number which can be expressed as a ratio of two integers.
that mean it can be express in a/b
if it can't, then it is not rational number

Rounding is the process of reducing the number of significant digits in a number. The result of rounding is a "shorter" number having fewer non-zero digits yet similar in magnitude. The result is less precise but easier to use.

Example: 73 rounded to the nearest ten is 70, because 73 is closer to 70 than to 80.

rounded value is the value after rounding