Respuesta :

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