So X squared is equal to two. So X must be the square root of two. Just using algebraic manipulation, we discover that for the A series of paper, the ratio of the longer side to the shorter side is the square root of two. Now, from your calculator, you will see that the square root of two has this decimal expansion and it goes on forever.
In Windows 10 Calculator first choose Scientific (topleft of the window), type in the number you want to square, then click the "2nd" button (topleft) and then the "2Vx" button below it.