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Not exactly. If I know the side lengths, I can still find the perimeter
You square-root the area
You cannot. For a given area, an equilateral triangle will have the smallest perimeter but that perimeter can be increased - without limit - without increasing the area.
If you know the area the divide area by length if you know perimeter subtract it by 2xlength and divide by 2
You have to add up all the sides of the shape.