Part (i)
We start with five dots to make the pattern in figure 1.
In figure 2, we add on 1 dot to each arm of the X shape. So that means we've added 4 dots total going from 5 to 5+4 = 9 dots.
In figure 3, there are 9+4 = 13 dots
So the pattern is simply "add 4" to get the next term. Again, this is because we add one dot per arm.
The first three terms of this arithmetic sequence are: 5, 9, 13
Your teacher wants to know what the general nth term is
We start with a = 5 and the common difference is d = 4
T(n) = nth term
T(n) = a + d(n-1)
T(n) = 5 + 4(n-1)
T(n) = 5 + 4n - 4
T(n) = 4n + 1
Let's try it out. Say we want to plug in n = 2
T(n) = 4n + 1
T(2) = 4(2) + 1
T(2) = 8 + 1
T(2) = 9
This works because the second figure indeed has 9 dots. I'll let you confirm the other figures.
Answer: 4n + 1
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Part (ii)
Your teacher wants to know how many dots occur when n = 50
T(n) = 4n + 1
T(50) = 4(50)+1
T(50) = 200 + 1
T(50) = 201
Verifying this through drawing dots is going to be a very tedious task, and I don't recommend it unless you really want to. Hopefully the verification process of T(2) = 9, and similar (for small values of n) is enough to convince you that this equation works as intended.
Answer: 201
