Question

Consider the following geometric sequence. -5, 10, -20, 40,... If the recursive formula for the sequence above is expressed in the form an=b(cn-1)nwhat is b and c

Answer 1

The formula of the geometric sequence is the following :
$u_n=bc^{n-1}$ wherein b is the first term and c is the rate. 
From the given data we deduce the parameters:
$c=\dfrac{10}{-5}=-2,b=-5$ 
Therefore:
$u_n=-5(-2)^{n-1}$