a1=b1=16
a3=16+2d
a6=16+5d
a3/a1=a6/a3=q
a3^2=a1*a6
(16+2d)^2=16(16+5d)
256+64d+4d^2=256+80d
4d^2-16d=0
4d(d-4)=0
d=4
16;20;24;28;32;36
24/16=3/2
36/24=3/2
a1=b1=16
a3=16+2d
a6=16+5d
a3/a1=a6/a3=q
a3^2=a1*a6
(16+2d)^2=16(16+5d)
256+64d+4d^2=256+80d
4d^2-16d=0
4d(d-4)=0
d=4
16;20;24;28;32;36
24/16=3/2
36/24=3/2