Sn=n(a1+an)/2=na1+n(n—1)d/2
Sn/n=a1+(n—1)d/2
S3/3=4+d,S4/4=4+3/2d S5/5=4+2d
(4+d)(4+3/2d)=(4+2d)(4+2d)
5/2d²+6d=0
d=-12/5
an=4—12/5(n—1)
Sn=n(a1+an)/2=na1+n(n—1)d/2
Sn/n=a1+(n—1)d/2
S3/3=4+d,S4/4=4+3/2d S5/5=4+2d
(4+d)(4+3/2d)=(4+2d)(4+2d)
5/2d²+6d=0
d=-12/5
an=4—12/5(n—1)