q=a1/a2=a3/a1
a1^2=a2*a3
a1^2=(a1+d)(a1+2d)
a1^2=a1^2+2a1d+a1d+2d^2
3a1d+2d^2=0
d(3a1+d)=0
d=0 не подходит
d=-3a1
q=a1/a2=a3/a1
a1^2=a2*a3
a1^2=(a1+d)(a1+2d)
a1^2=a1^2+2a1d+a1d+2d^2
3a1d+2d^2=0
d(3a1+d)=0
d=0 не подходит
d=-3a1