You can put this solution on YOUR website! Is it 40 ft on a side? that's a big tent.
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Or 40 ft across the diagonal?
If it's 40 across the diagonal, it's 6 equilateral triangles with 20 foot sides.
Area of each triangle is 20*10*sqrt(3)/2
Total area = 600*sqrt(3) =~1039.23 sq feet
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For 40 feet on a side, it's 4 times that.