Question

if $a_1 = 10$ and $a_n = -2a_{n - 1}$ then find the value of $a_4$.

Explanation:

Step1: Find $a_2$ using $a_1$

$a_2 = -2a_1 = -2\times10 = -20$

Step2: Find $a_3$ using $a_2$

$a_3 = -2a_2 = -2\times(-20) = 40$

Step3: Find $a_4$ using $a_3$

$a_4 = -2a_3 = -2\times40 = -80$

Answer:

$-80$