請問 年利率問題

23歲生日存入的$10000到25歲生日得$10000*(1 + r/100)2
24歲生日存入的$9000到25歲生日得$9000*(1 + r/100)
25歲生日得到$22000 = $10000*(1 + r/100)2 + $9000*(1 + r/100)
22 = 10(1 + r/100)2 + 9(1 + r/100)
Let x = r/100
22 = 10(1 + x)2 + 9(1 + x)
22 = 10 + 20x + 10x2 + 9 + 9x
10x2 + 29x - 3 = 0
(10x - 1)(x + 3) = 0
x = 0.1 or x = -3(不合)
x = 0.1 => r = 10

[10000x(1+r%)+9000]x(1+r%)=22000
(10000+100r+9000)x(1+r%)=22000
(19000+100r)x(1+r%)=22000
19000+100r+190r+r2=22000
r2+290r-3000=0
r=10 或 r=-300(捨去)

2009-09-17 21:21:04 補充：
複利息？