In an A.P, a7=9  and if a1a2a7  is least then common difference is

given that a7=a+6d=9,a1a2a7=a(a+d)(a+6d)  =9-6d 9-6d+d9=9-6d9-5d9  =273-2d9-5d  =2710d2-33d+27 =270d2-3310d+2710 =270d-33202-1089400+2710=270{(d−3320)2−9400}  is least when d=3320