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A stone is thrown vertically downwards and the formula d = 5t^2 + 25t gives the distance, d metres, that it falls in t seconds. How long does it take to fall 1500 metres



Answer :

Answer:

use the formula we know the distance is 1500 metres

put that in the formula

1500 = 5t^2 + 25t

Now we can factor out 5 as all the coefficients are multiples of 5

so

we get

300 = t^2 +5 t

now we solve for t using the quadratic factorization

t^2 +5t -300 = 0

which gives

(t - 15) =0

(t + 20 )= 0

so time cannot be negative hence

15 seconds is the answer

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