0.99... = X

양변에 10을 곱 해준다

10 * 0.99.. = 10 * X

9.999.. = 10 X

9 + X = 10 X

9 = 10 X - X

9 = 9 X

1 = X

=============================================================================

0.99... = X

양변에 10을 곱 해준다

10 * 0.99.. = 10 * X

9.999.. = 10 X

9 + X = 10 * 0.99...

9 + X = 9.99...

X = 9.99... - 9

X = 0.99....