Area of a kite is half the product of the diagonals: (d₁ × d₂) / 2. A rhombus uses the same diagonal formula, in area of a rhombus.
Diagonals, not four sides
Side length alone is not the area. Similar solids are a different leftover, in similar solids. Area and volume as a set are in area and volume.
The bottom line
(d₁d₂)/2. You need both diagonals.
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