# If 5, x, y, z, 1280 are in GP, what are the values of x, y, z.

$\begin{array}{1 1} -20 ,-80,-120 \\ \pm 20,80 ,\pm 120 \\ 16,64,256 \\ 320 ,-80,20 \end{array}$

Answer : $(b) \pm20,80,\pm320$
Explanation : $In \;a\; GP ,a=5$
$ar^4=1280\qquad\;r^4=\large\frac{1280}{5}=256$
$r^2=16\qquad\;r=\pm4.$
$When\; r=+4\;,x,y,z=20,80,320$
$When\; r=-4\;,x,y,z=-20,80,-320$
edited Jan 24, 2014