How do you know when to use 3.14 or 3 when using pi?

Answer 1

(Pi) has an infinite number of decimal places, and never ends. So if you use 'pi' to get

the answer to a problem, then the answer must always be approximate; it can never

be exactly true.

The more decimal places of 'pi' you use when you work the problem, the more accurate

your answer will be.

Here's a tiny portion of the beginning of 'pi'. Use as many or as few decimal places

as you want.

3.14159 26535 89793 23846 26433 83279 50288 . . . . .

However many decimal places you use will be the same as the number of digits in

your answer that are correct. After those, none of the rest mean anything, and

you should get rid of them by rounding the answer.