Answer: 4n-3
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Explanation:
day 1 = 1 sold
day 2 = 5 sold
day 3 = 9 sold
day 4 = 13 sold
Each time we're going up by 4. This is effectively the slope of the line since it represents the rate of change. The starting amount sold (1) is the y intercept
So we go from y = mx+b to y = 4x+1
Afterward, replace x with n to get 4n+1
On day n = 0, we have 4n+1 = 4(0)+1 = 1 sold. While this is correct, it doesn't really help to start with day 0 and it's better to start with day 1 instead.
What we can do is replace n with (n-1) and then simplify
4(n-1)+1 = 4n-4+1 = 4n-3
Now plugging in n = 1 leads to 4n-3 = 4(1)-3 = 1
Plugging in n = 2 leads to 4n-3 = 4(2)-3 = 5
and so on.
