Hi there!
v = ±
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K = 1/2mv²
Isolate for the variable "v". We can begin by dividing both sides by 1/2. (Multiply by the reciprocal, or 2):
2 · K = 2 · (1/2mv²)
2K = mv²
Continue isolating by dividing both sides by "m":
2K / m = v²
Take the square root of both sides. Remember that the solution can either be positive or negative since there are positive and negative roots.
v = ±