Answer:
common ratio is -3 or 3
Step-by-step explanation:
[tex]u_1=1\\\\u_3=u_1*r^2=r^2\\\\u_5=u_1*r^4=r^4\\\\r^2+r^4=90\\\\r^4+r^2-90=0\\\Delta=1+4*90=361=19^2\\\\r^2=\dfrac{-1-19}{2} =-10 (exclude\ a\ square\ is\ positive)\\or\\r^2=\dfrac{-1+19}{2}=9\\r^2-9=0\\(r-3)(r+3)=0\\So\ r=-3\ or\ r=3\\[/tex]
