5
21
isosceles triangle
A chord is a straight line drawn through a circle which divides the circle into two parts. The line can be drawn anywhere in the circle EXCEPT the center where it becomes the diameter.
There are infinite diameters within a circle.
general astronomy
If you mean can a circle always be drawn round a quadrilateral so that the quadrilateral is enclosed within the circle then yes as long as the diameter of the circle is large enough. If you mean can a circle always be drawn around a quadrilateral so that it passes through all vertices then only if the opposite angles of the quadrilateral add up to 180o - such quadrilaterals are called cyclic quadrilaterals.
21
Use the formula n(n-1)/2 --> 7(7-1)/2 = 7(6)/2 = 42/2 = 21.
The answer is 1225.
Chord.
A straight line segment can be drawn joining any two points.
66 ( 12 nCr 2 )
A circle is the locus of all points equidistant from a given point, which is the center of the circle, and a circle can be drawn with a compass. (The phrase "locus of points for a circle" does not seem to be conventionally defined.) or true
All points on the circumference of a circle drawn on a plane are equidistant from the single point on the plane which is the center of the circle.
isosceles triangle
The line joining the feet of the perpendiculars drawn from all the points of the line onto a preselected plane.
The circle doesn't fit because it is a curve. The other figures are drawn with line segments; they're polygons. In fact, the other figures are all quadrilaterals.