一條中4quadratic既應用題..超急!!

let the original of the car is x km/h

240/x - 240/(x+15) = 4/5

(240x + 3600 - 240x ) / (x^2 + 15x ) = 4/5

18000 = 4x^2 + 60x

x^2 + 15x - 4500 = 0

(x-60) (x+75) = 0

x = 60 or x = -75 (rejected)

Therefore , the original of the car is 60km / h

解釋:

240/x 係原本架車要行既時間 ...
240 / (x+15) 係新既時間 ...

而左邊就係將舊減個新既時間 ....
右邊就係新車速慳左落黎既 4/5 hour

我都唔知有冇計錯 ... 希望幫到你 !!