A(n) is a letter or symbol that represents some unknown value.
a. term
b. equation
c. variable
d. expression

It is a variable

Step-by-step explanation:

A variable is a characteristic, number, or quantity that increases or decreases over time or takes different values in different situations. Hope this helps.

, and

for

step-by-step explanation:

we can analyze the pattern at which the terms increase. let us denote , , , so that we can call

note the following:

the pattern: to get the next term value, we just add the next term number (e.g. 1st, 2nd, to the previous term value (term values are the given numbers in the question, 1, 3, 6, 10, 15,

from , to get to , we add

from , to get to , we add

etc.

we need to form a recursive formula, a formula that has the next term as a function of the previous term. so a possible recursive formula can be

, and for