Answer:
[tex] \frac{3}{2} n {}^{2} + \frac{9}{2} n - 12[/tex]
Step-by-step explanation:
This sequence is a quadratic sequence meaning that we should find the difference and the constant second difference.
The difference between the 4 terms is in the sequence:
9;12;15
The constant second difference is 3.
Use your method of finding the general term:
2a=3 3a+b=9 a+b+c=-6
a=3/2 3(3/2)+b=9 3/2+9/2+c=-6
b=9/2 c=-12
Standard form for quadratic sequence:
[tex] {an}^{2} + bn + c[/tex]
Therefore your general term is the one stated above in the answer.
Hope that helps!
