For a particle moving along a straight line path, the displacements in third and fifth seconds of its motion are 10 m and 18 m respectively. The speed of the particle at time t=4s is
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The formula for displacement in the $n^{th}$ second, $s_n = u + a(n-1/2)$, is very useful for problems involving specific time intervals. It is derived from the difference in positions at time $n$ and $n-1$: $s(n) - s(n-1)$.