解:?$(2)$?設(shè)?$x = 0.\dot {4},$?①
?$10x = 10×0.\dot {4},$?②
?$10x = 4.\dot {4},$?③
?$10x = 4 + 0.\dot {4},$?④
?$10x = 4 + x,$?⑤
?$9x = 4,$?⑥
?$x = \frac {4}{9}。$?⑦
設(shè)?$y = 2.\dot {5},$?①
?$10y = 10×2.\dot {5},$?②
?$10y = 25.\dot {5},$?③
?$10y = 23 + 2.\dot {5},$?④
?$10y = 23 + y,$?⑤
?$9y = 23,$?⑥
?$y = \frac {23}{9}。$?⑦