A3*a6=(a1+2d)(a1+5d)=a1²+2da1+5da1+10d²=a1²+7a1*d+10d²
a1²+7a1*d+10d²=406
2a4+6=a9 2(a1+3d)+6=a1+8d 2a1+6d+6=a1+8d
a1-2d=-6 a1=2d-6
4d²-24d+36+7(2d-6)d+10d²=4d²-24d+36+14d²-42d+10d²=
= 28d²-66d+36=406
14d²-33d+18=203 14d²-33-185=0
D=33²+4*14*185=1089+10360=11449 √D=107
d1=1/28[33+107]=140/28=5 d2=1/28[33-107]= -74/28=-37/14
d=5 a1=2*5-6=4
d=-37/14 a1=-37/7-6=-11 2/7