Answer:
3n² + 5n - 2
Step-by-step explanation:
<u>Given sequence</u>:
6, 20, 40, 66, 98, 136, ...
Calculate the <u>first differences</u> between the terms:
![6 \underset{+14}{\longrightarrow} 20 \underset{+20}{\longrightarrow} 40 \underset{+26}{\longrightarrow} 66 \underset{+32}{\longrightarrow} 98 \underset{+38}{\longrightarrow} 136](https://tex.z-dn.net/?f=6%20%5Cunderset%7B%2B14%7D%7B%5Clongrightarrow%7D%2020%20%5Cunderset%7B%2B20%7D%7B%5Clongrightarrow%7D%2040%20%5Cunderset%7B%2B26%7D%7B%5Clongrightarrow%7D%2066%20%5Cunderset%7B%2B32%7D%7B%5Clongrightarrow%7D%2098%20%5Cunderset%7B%2B38%7D%7B%5Clongrightarrow%7D%20136)
As the first differences are not the same, calculate the <u>second differences:</u>
![14 \underset{+6}{\longrightarrow} 20 \underset{+6}{\longrightarrow} 26 \underset{+6}{\longrightarrow} 32 \underset{+6}{\longrightarrow} 38](https://tex.z-dn.net/?f=14%20%5Cunderset%7B%2B6%7D%7B%5Clongrightarrow%7D%2020%20%5Cunderset%7B%2B6%7D%7B%5Clongrightarrow%7D%2026%20%5Cunderset%7B%2B6%7D%7B%5Clongrightarrow%7D%2032%20%5Cunderset%7B%2B6%7D%7B%5Clongrightarrow%7D%2038)
As the <u>second differences are the same</u>, the sequence is quadratic and will contain an n² term.
The <u>coefficient</u> of the n² term is <u>half of the second difference</u>.
Therefore, the n² term is: 3n²
Compare 3n² with the given sequence:
![\begin{array}{|c|c|c|c|c|}\cline{1-5} n & 1 & 2 & 3 & 4\\\cline{1-5} 3n^2 & 3 & 12 & 27 & 48 \\\cline{1-5} \sf operation & +3&+8 & +13 & +18 \\\cline{1-5} \sf sequence & 6 & 20 & 40 & 66\\\cline{1-5}\end{array}](https://tex.z-dn.net/?f=%5Cbegin%7Barray%7D%7B%7Cc%7Cc%7Cc%7Cc%7Cc%7C%7D%5Ccline%7B1-5%7D%20n%20%26%201%20%26%202%20%26%203%20%26%204%5C%5C%5Ccline%7B1-5%7D%203n%5E2%20%26%203%20%26%2012%20%26%2027%20%26%2048%20%5C%5C%5Ccline%7B1-5%7D%20%5Csf%20operation%20%26%20%2B3%26%2B8%20%26%20%2B13%20%26%20%2B18%20%5C%5C%5Ccline%7B1-5%7D%20%5Csf%20sequence%20%26%206%20%26%2020%20%26%2040%20%26%2066%5C%5C%5Ccline%7B1-5%7D%5Cend%7Barray%7D)
The second operations are different, therefore calculate the differences <em>between</em> the second operations:
![3 \underset{+5}{\longrightarrow} 8 \underset{+5}{\longrightarrow} 13\underset{+5}{\longrightarrow} 18](https://tex.z-dn.net/?f=3%20%5Cunderset%7B%2B5%7D%7B%5Clongrightarrow%7D%208%20%5Cunderset%7B%2B5%7D%7B%5Clongrightarrow%7D%2013%5Cunderset%7B%2B5%7D%7B%5Clongrightarrow%7D%2018)
As the differences are the same, we need to add 5n as the second operation:
![\begin{array}{|c|c|c|c|c|}\cline{1-5} n & 1 & 2 & 3 & 4\\\cline{1-5} 3n^2 +5n & 8&22 & 42 & 68\\\cline{1-5}\sf operation & -2 &-2 &-2 & -2 \\\cline{1-5} \sf sequence & 6 & 20 & 40 & 66\\\cline{1-5}\end{array}](https://tex.z-dn.net/?f=%5Cbegin%7Barray%7D%7B%7Cc%7Cc%7Cc%7Cc%7Cc%7C%7D%5Ccline%7B1-5%7D%20n%20%26%201%20%26%202%20%26%203%20%26%204%5C%5C%5Ccline%7B1-5%7D%203n%5E2%20%20%2B5n%20%26%208%2622%20%26%2042%20%26%2068%5C%5C%5Ccline%7B1-5%7D%5Csf%20operation%20%26%20-2%20%26-2%20%20%26-2%20%20%26%20-2%20%20%5C%5C%5Ccline%7B1-5%7D%20%5Csf%20sequence%20%26%206%20%26%2020%20%26%2040%20%26%2066%5C%5C%5Ccline%7B1-5%7D%5Cend%7Barray%7D)
Finally, we can clearly see that the operation to get from 3n² + 5n to the given sequence is to subtract 2.
Therefore, the nth term of the quadratic sequence is:
3n² + 5n - 2