An convex n-gon has n(n-3)/2 diagonals.
So a square is a 4-gon and it has 4(1)/2=2 diagonals.
A pentagon has 5(2)/2=5 diagonals.
quadrilaterals always have 2 diagonals.
Here is a link with some pics to help you see the various kinds they have. That is to say, are they perpendicular, to they bisect each other?
http://illuminations.nctm.org/LessonDetail.aspx?id=L655
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A parallelogram has two diagonals the same as all 4 sided quadrilaterals
There are lot of shapes in geometry. It is called polygons. Triangle, trapezoid, circle, parallelogram, rhombus, rectangle, diagonal, square etc. Parallelogram, rhombus, rectangle, diagonal, square, and trapezoid are quadrilaterals.
Not always. They would only intersect when the internal angles are each less than 180 degrees - which not all quadrilaterals have.
2 diagonals bisect each other only in the case of square , parallelogram, rhombus , rectangle and isosceles trapezium ;not in ordinary quadrilaterals.
The sum of all (interior) angles must equal 360 degrees