Answer:
add the areas of the parts
Explanation:
The area of an unusual shape is often easily found by decomposing it into shapes that you have formulas for. Divide the shape into parts, and add the areas of those parts.
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negative area
Alternatively, some shapes are conveniently described by "missing" parts, For example, your "plus shaped" figure may be described as an overall rectangle with rectangles removed from the corners. Then the area of the shape is found by subtracting the missing area from that of the overall rectangle.
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overlapping parts
As a rule, you want the parts to be non-overlapping. That way, you can add the areas of the parts without counting any area twice (or more times). If you have overlapping parts, you need to subtract the area(s) that may be counted multiple times.
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symmetry
Sometimes you can take advantage of symmetry, so that you only need to compute part of the total area, then multiply by a factor that takes the symmetry into account.
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example
In the attached figure, the dimensions of rectangle A are 6 m wide by 7 m high. The dimensions of rectangle B must be found by adding the width dimensions shown across the top of the figure: 5 m +6 m +6 m = 16 m. It is marked as 4 m high. Then the total area is ...
area of A + area of B
= (6 m)(7 m) +(16 m)(4 m)
= 42 m² +64 m² = 106 m²
This can also be found as the area of the 16 m by 11 m enclosing rectangle, less the two 5 m by 7 m corner cut-outs.
(16 m)(11 m) -2(5 m)(7 m) = 176 m² -2(35 m²) = 106 m²