Question

I do not understand what subscript numbers mean

Answer 1

subscripts identify different instances of objects that have a similar meaning or definition

Explanation:

You apparently are interested in the meaning of the subscript N in the explicit formula ...

`A_N=(7N)/(N+5)`

Here, it is used to identify different terms of a sequence. When a value is substituted for N, it replaces all instances of N. Using your example, the first three values of
`A_N` are ...

`\text{for N=1: }A_1=(7\cdot 1)/(1+5)=(7)/(6)\\\\\text{for N=2: }A_2=(7\cdot 2)/(2+5)=(14)/(7)=2\\\\\text{for N=3: }A_3=(7\cdot 3)/(3+5)=(21)/(8)`

So, if you're asked to find the 20th term of the sequence, you put 20 where N is, then do the arithmetic.

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Subscripts are also used to identify elements of a matrix or coordinates in multiple dimensions. For this purpose, there may be 2 (or more) subscripts, whose order has meaning. The subscripts 24, for example, in
`A_(24)` refer to row 2, column 4 of a matrix named A. That is, the subscripts are in row, column order.

In short, subscripts are used to identify different instances of similar objects.