可以幫我解答這題中四數學應用題嗎???

兩年前價 * r% = 去年價....(1)

去年價 * (r - 5)% = 現價....(2)

(1)代入(2) :

兩年前價 * r% * (r - 5)% = 現價...(3)

現時的價值是兩年前的68%,即:

兩年前價 * 68% = 現價...(4)

(4)比較(3)即

r% * (r - 5)% = 68%

r/100 * (r - 5)/100 = 68/100

r(r - 5) = 6800

r^2 - 5r - 6800 = 0

(r - 85)(r + 80) = 0

r = 85 或 r = - 80(捨去)



2009-12-25 15:57:50 補充：
*是代表乘,thx!