Part 1: Serena's drawing Follow these steps to find the angles of the trapezoids in Serena's drawing. If the angles of her
trapezoid match the angles of the trapezoid stone, then her drawing is correct.

BCDE= 100+ 80+100+80 = 360 degree

BAC = 20 degree

DBC = 100 degree

BDE = 80 degree

Shows all is correct

We associate isosceles trapezoid in this question, this helps us prove with angle A in the triangle vertices how we can find one of the adjacent angles from the trapezoid by subtraction when all angles within one single trapezoid = 360 degrees. We can use 20 degree angle A to subtract and find each alternative angle easier.

Step-by-step explanation:

The semi circle shows 9 stones

Where angles are interior to the trapezoid

Where all 4 sided shapes add up to 360 degree

We draw a line of symmetry on BCED angle

To make midway points BC  and DE = 90 degree

The two new formed shapes are still 4 sided and add up to 360 degree.

BC + DE = 180 degree

BDE = Triangle 180 -20/2 = 160/2 = 80

BCDE = Trapezoid = 360

Trapezoid angle DBC = 360-80-80/2 = 360-160/2 = 200/2 = 100 degree  

Finding interior angle A (BAC)

BAC = 180/9 = 20 degree

BAC * (2) = 360 degree circle

BAC = 360/18 = 20 degree

Proves BAC = 20 degree

20 degree is used in BAC workings above in bold and proves all trapezoid angles are correct. We now know this is an isosceles trapezoid and that is why symmetry and midway points can help us find the angles without any given length.

b is 38 degrees.

a is 52 degrees.

this is because if b is 38 and a is 14 degrees more than b than 38+14=52 and 52+38=90 degrees. hope this .