I need help with these four off my review sheet. I would love some explanations

1. Find the specified term of the arithmetic sequence:

t2=9, t5=21; 41=__

2. Find the specified term of the geometric sequence:

t2=8,t4=72; find t6=__

3. Find the sum of the arithmetic series. If there is no sum, state so.

50+48+46+ ... +10

4. Sorry im not sure how to type this question so try to bare with me lol

Lim x-->infinity (radical(1+x)-1)/(x)