Question

What is the explicit formula that represents the geometric sequence -2, 8, -32, 128

Answer 1

$a_n=a_1r^{n-1}\\\\a_1=-2;\ a_2=8;\ a_3=-32;\ a_4=128\\\\r=a_2:a_1\to r=8:(-2)=-4\\\\\boxed{a_n=-2\cdot(-4)^{n-1}}$