If the blue radius below is perpendicular to the green chord and the segment
ab is 9 units long, what is the length of the chord?

There's the obvious symmetry ultimately because the radii OA and OB are hypotenuses of congruent right triangles.  We can just eyeball the figure and see AB=BC, at least approximately, so the full chord AC=9+9=18

A. 18 units

for ex: the reciprocal of 3/4 is 4/3. it is the opposite.

reciprocal definition:

a mathematical expression or function so related to another that their product is one; the quantity obtained by dividing the number one by a given quantity.

a net is a diagram that shows a “flattened” view of a solid. in a net, each face and base is shown with all of its dimensions. a net can also serve as a pattern to build a three-dimensional solid.

a pyramid is a three-dimensional object with a base that is a polygon and triangular faces that meet at one vertex.

" the net of a triangular pyramid consists of four triangles. the base here we are given of the pyramid is a equilateral   triangle, and the lateral faces are also triangles "