A ball is thrown straight up, and its height after t seconds is h(t) = 20t - 5t^2 metres. What is its greatest height?
Answer and explanation
Correct answer: B. 20 m
- Step 1: h'(t) = 20 - 10t = 0 gives t = 2 s. Step 2. h(2) = 40 - 20 = 20 m. Answer: 20 m. Why not the others: 2 is the time in seconds at the top, not the height, 40 m leaves out the -5t^2 term, and 15 m is the height after 1 second.
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