Derive newtons first law of motion from second law.

Newton's second law states that F=ma.
Assuming that no external force is being applied on the body, i.e., F=0,
0 = ma
=> 0 = m [(v-u)/t]
=> 0/m = (v-u)/t
=> 0*t = v-u
​=> 0 = v-u
=> u = v
According to Newton's first law of motion, a body moving with a certain velocity continues to move with the same velocity, i.e., u=v; until and unless an external force is applied, which may change the speed of the body.
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