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A4-Diagonal
In the A-series paper size, the ratio of width, length and diagonal is √1 : √2 : √3.
Explanation
If you cut a diagonal in a sheet of A4 paper, its length is √3.
√2 √3 √1
Indeed, the Pythagorean theorem gives here
(√1)2 + (√2)2 = (√3)2