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(20-2)x180/20=162
Two
You don't - you need additional information. Many different rectangles can have the same diagonal. If you know the diagonal and one side (which must be LESS than the diagonal), you can use Pythagoras' Theorem to calculate the other side.
9 diagonals
36
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It has 1,345,890 diagonals
triacontakaihexagon
The sum of the internal angles of any regular polygon is (n - 2) x 180 degrees. Since a triacontakaihexagon has 36 sides, the sum of the internal angles of a triacontakaihexagon has a sum of internal angles of (36 - 2) x 180 = 6120 degrees.
A diagonal cannot be a side of a rectangle, and a side cannot be a diagonal.
There are four diagonal lines in a hexagon.
(20-2)x180/20=162
A heptagon has 4 diagonal lines and 7 sides and it has 2 congruent diagonal lines and 2 congruent triangles
Not many. Depending on how you write, the letters L, O, Q, and X can have diagonal lines of symmetry.
4
zero