Suppose that a particle is sitting at the origin, and, at t = 0 s, starts moving at 3 m s−1 away from the origin for 5 seconds.

Then, if we let d denote the distance (in metres) of the particle to the origin, we know that d = 3t, where t is the time in seconds. Now suppose that at time t = 5 s the particle starts moving away from the origin at speed 6 m s−1

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Write d as a function of t.