A quadtriatel is said to contain perpendicular diagnols if four go 90 degree angles are formed at the intersection of these lines
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Your sequence appears to be geometric with a common ratio of 2. It can be described by
a(n) = (-2 2/3)·2^(n-1)
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This can be written in a number of other forms, including
a(n) = (-8/3)·2^(n-1)
a(n) = (-1/3)·2^(n+2)
a(n) = (-4/3)·2^n
The fraction is 27/30. It is equivalent to 9/10 and 27 + 30 = 57.
