Answer:

Step-by-step explanation:
Given


Required
The explicit formula
First, calculate 
Set n=2; So:


Substitute -25 for 


Calculate the common difference (d)



So, the nth term is:


Open bracket

Collect like terms


Answer:
The estimated sum is ≈10
Step-by-step explanation:
This depends on how your professor wants you to estimate sums. Logically, though, a value of nearly 4 and a value of just over 6 add to about 10. The exact answer would be a bit over 10, at ≈10.14117
Answer:
Step-by-step explanation:
What diagram?
A parallelogram
Because...
-opposite angles are congruent
-four sides
-angles must equal 360
100 + 100 +80 + y = 360
280 + y = 360
y = 80
-the angle we don't know is 80 degrees