Arithmetic Geometric Series and Sequences

Which kind of sequence has a common difference?

Is this sequence arithmetic, geometric, or neither?
{26, 22, 18, 14, ...}

What kind of sequence is illustrated below?
 400, 200, 100, 50, 25, ...

Is the sequence arithmetic: 78, 785, 7855, 78555, ...

Determine if the situation can be represented by an arithmetic or geometric sequence. Seeds cost $1.20 be packet. The cost for 1 packet, 2 packets, 3 packets and so on. ​ (a)   The value of a baseball card is increasing by 2% every year. ​ (b)   Interest on a Student Loan is 3% compounded bi-annually. ​ (c)   The first 3 terms in the sequence are $$3,-9,27...$$ ​ (d)   You earn $20 a week for allowance. ​ (e)  

Calculate the sum of the first 15 terms of the arithmetic sequence where $$a_1 = 7$$ and $$d = 3$$ .

Evaluate the Arithmetic Series

Find the sum of the infinite geometric series, if it exists:

5 + 4 + 3.2 + 2.56 + ...

Determine whether the sequence could be geometric or arithmetic:

6, 10, 15, 21, ...

What is the sum of the first 10 terms for the following geometric series:
2-6+18-54+...