0.181818… = 18 (0.010101…)
… = 18 (0.01 + 0.0001 + 0.000001 + …)
… = 18 (1/100 + 1/100² + 1/100³ + …)
… = 18 (1 + 1/100 + 1/100² + 1/100³ + …) - 18
Then you have
[tex]0.181818\ldots = \displaystyle18\sum_{k=0}^\infty\frac1{100^k} - 18 = \frac{18}{1-\frac1{100}} - 18 = \boxed{\frac2{11}}[/tex]
