Respuesta :
Check the forward differences of the sequence.
• first-order differences
5 - 1 = 4
12 - 5 = 7
22 - 12 = 10
35 - 22 = 13
• second-order differences (i.e. differences of the first differences)
7 - 4 = 3
10 - 7 = 3
13 - 10 = 3
The second differences are all 3 (as far as we know), so the sequence of first differences is arithmetic/linear, which means the original sequence is quadratic. Let the [tex]n[/tex]-th term be
[tex]x_n = an^2 + bn + c[/tex]
Given that [tex]x_1=1[/tex], [tex]x_2=5[/tex], and [tex]x_3=12[/tex], we have
[tex]\begin{cases} a + b + c = 1 \\ 4a + 2b + c = 5 \\ 9a + 3b + c = 12 \end{cases} \implies a=\dfrac32, b=-\dfrac12, c=0[/tex]
and so the [tex]n[/tex]-th term of the sequence is generated by the rule
[tex]x_n = \dfrac{3n^2 - n}2[/tex]
which most closely resembles the last option,
Square the term number, multiply the result by 3, subtract the term number, and divide the result by 2.
