9514 1404 393
Answer:
- next term: 15
- an = n(n+1)/2
Step-by-step explanation:
Each term is the previous term with the current term number added. The 5th term will be 5 + (4th term) = 5 + 10 = 15.
The next term is 15.
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The sequence of triangular numbers is given by the explicit formula ...
an = n(n+1)/2
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Finding the Formula
The sequence of first differences is ...
3 -1 = 2
6 -3 = 3
10 -6 = 4
And the sequence of second differences is ...
3 -2 = 1
4 -3 = 1
These are constant, indicating the sequence is described by a polynomial function of 2nd degree.
There are several ways to find the coefficients of the quadratic function. One is to solve for them using a system of equations.
n = 1
a1 = 1 = a(1)^2 +b(1) + c
n = 2
a2 = 3 = a(2)^2 + b(2) +c
n = 3
a3 = 6 = a(3)^2 +b(3) +c
To solve, we can subtract each equation from the next:
a2 -a1 = (3) -(1) = a(4 -1) +b(2 -1) +c(1 -1)
2 = 3a +b
and
a3 -a2 = (6) -(3) = a(9 -4) +b(3 -2) +c(1 -1)
3 = 5a +b
Now we can subtract the first of these equation from the second:
(3) -(2) = a(5 -3) +b(1 -1) ⇒ 1 = 2a ⇒ a = 1/2
Then ...
2 = 3(1/2) +b
1/2 = b
1 = a + b + c ⇒ 1 = 1/2 + 1/2 + c ⇒ c = 0
So, the quadratic function describing the sequence of triangular numbers is ...
an = (1/2)(n^2 +n)
an = n(n+1)/2
