In arithmetic sequence tn find:

Question

t5 , if t7=−20 and S8 = −100
t5 =

Answer 1

the common difference is just t8-t7 = -80
Now t5 = t7 - 2d = -20 - 2(-80) = 140

Answer 2

It's sum of 8 not T8

Answer 3

oops. well, just use the right formulas, which I'm sure you know
a+6d = -20
8/2 (2a+7d) = -100
solve for a and d, and then t5 = a+4d