(2) 设x=0.021212121212121……
两边同乘以10得, 10x=0.21.....
两边同乘以1000得, 1000x=21.21....
1000x=21+0.21...
1000x=21+10x
990x=21
x=21/990
所以:2.521212121212121…… =2.5+x=5/2+21/990=2496/990
(3) 甲的费用=30×100=3000
乙的费用=31×95=2845
设有x名学同参加.
100x=95(x+1)
解得: x=19
(2) 设x=0.021212121212121……
两边同乘以10得, 10x=0.21.....
两边同乘以1000得, 1000x=21.21....
1000x=21+0.21...
1000x=21+10x
990x=21
x=21/990
所以:2.521212121212121…… =2.5+x=5/2+21/990=2496/990
(3) 甲的费用=30×100=3000
乙的费用=31×95=2845
设有x名学同参加.
100x=95(x+1)
解得: x=19