F(t) = -5t^2 + 20t + 60
-5(t^2 - 4t - 12)
-5(t + 2) (t - 6)
t = -2 or t = 6
6 seconds
<em>Denote x2 by y.
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<em>(x2-3)7=(y-3)7
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<em>This is a binomial expansion in y, and you want the coefficient of y4 because y4=x8
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<em>You have 7 terms of (y-3) in (y-3)7. To get the fourth power of y, you need to choose y from four of the terms. The number of ways you can do this is the combinations of 7 things taken 4 at a time. This is:
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<em>7!/(4!3!)=35</em>
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<em>So, the coefficient of x8 in the given expansion will be 210.</em>
<em>HOPE IT HELPS</em>
<em>THANK YOU </em>
