a) the time the stone took to reach the ground again;
b) the greatest height reached;
c) the average speed of the stone during the flight.
更新1:
可以解釋一下a)嗎??不是很明白...
maths...
2006-10-15 2:11 am
1. a stone was thrown vertically upwards from the ground. Its height above the ground, y metres, reached in x seconds was given by y=20x-5x^2. Find
a) the time the stone took to reach the ground again;
b) the greatest height reached;
c) the average speed of the stone during the flight.
a) the time the stone took to reach the ground again;
b) the greatest height reached;
c) the average speed of the stone during the flight.
回答 (1)
2006-10-15 2:27 am
✔ 最佳答案
a)y=0 when the stone reaches the ground
ie. 20x-5x^2 = 0
5(4-x)x = 0
x = 0 (rejected) or x = 4
Therefore, it reaches the ground in 4 seconds
b)
y = 20x-5x^2
= 5(4x-x^2)
= 5[-(2)^2+4x-x^2]+20
= -5(x-2)^2+20
Therefore, the greatest height = 20m when x = 2s
c)
distance travelled = 20 * 2 = 40m
time travelled = 4s
Therefore, average speed = 40/4 = 10 m/s
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