Question

What is the 9th term of the sequence 2, - 6, 18,- 54, ...?

Answer 1

$a_1=2;\ a_2=-6;\ a_3=18;\ a_4=-54\\\\It's\ a\ geometric\ sequence:a_n=a_1r^{n-1}\\\\r=a_2:a_1\to r=-6:2=-3\\\\a_n=2\cdot(-3)^{n-1}\\\\n=9\to a_9=2\cdot(-3)^{9-1}=2\cdot(-3)^8=2\cdot6,561=13,122\\\\\boxed{a_9=13,122}$