1題F.4數學題

Let v km/h be the original speed of the car.

time = distance/speed

Time required for the original speed
= 240/v h

Time required when the speed is increased by 15 km/h
= 240/(v+15) h

80 min = (80/60) h

An equation is set up:
(240/v) - [240/(v+15)] = (80/60)

Multiply both sides by 3v(v+5):
720(v+15) - 720v = 4v(v+15)
180(v+15) - 180v = v(v+15)
180v + 2770 - 180v = v2 + 15v
v2 + 15v - 2700 = 0
(v - 45)(v + 60) = 0
v = 45 or v = -60 (rejected)

Ans: The original speed is 45 km/h.
=

你可以去問問你的數學科老師.