Find the surface area of the figure: A square pyramid measuring 6 mi along the base with a slant height of 10.4 mi

Part A:

From the appearance of the figure above, triangle A B C forms a right angle at B with A B and B C being the legs and A C being the hypothenuse.

Given that A B is 20 and B C is 21, by the pythagoras theorem,

Part B:

From the appearance of the figure above, triangle A B C forms a right angle at B with A B and B C as the legs and A C as the hypothenuse.

If this is true, then the measure of angle A B C is 90 degrees.

Part C:

If A E is 10 and A F is half of A C, the special name for segment E F as it relates to triangle A B C is the midsegment.

The midsegment of a triangle is a line segment which joins the midpoints of two sides of the triangle.

Part D:

Given from part C above that line segment EF is a midsegment of triangle ABC, from the triangle midsegment theorem, line segment EF is parallel to side BC.

Thus, line BF is a transverse of parallel lines EF and BC which makes angle EFB alternate to angle FBC.

Since alternate angles are equal, given that angle EFB is 43.6°, then the measure of angle FBC  is also 43.6° because they are alternate angles.

Part E:

If Triangle DCB is congruent to triangle ABC, then angle B is congruent to angle C.

Given from part B that angle B = 90 degrees, then angle C = 90 degrees. Thus, the two adjacent angles of the quadrilateral ABCD = 90 degrees. Whih shows that the shape ABCD is a rectangle.