Answer:
Step-by-step explanation:

I can't read that language but I'll guess it says write a second degree equation then solve for n when d=10.



Answer: n² - 3n - 20 = 0
That doesn't factor so there is no integer n solution.
That means there are no polygons with 10 diagonals.

Answer:
28
Step-by-step explanation:
You multiply 7 and 8, and then divide it by two.
Answer: 
Step-by-step explanation:
All that is required here is to substitute x = 2x into the right side in the same way that is done for a numeric value.

C, 16cm. Count along the edges for the number of lines.