已只等差数列{an}中,an=3n-5,求其前n项和公式及s20

已只等差数列{an}中,an=3n-5,求其前n项和公式及s20
已只等差数列{an}中,an=3n-5,求其前n项和公式及s20

已只等差数列{an}中,an=3n-5,求其前n项和公式及s20
a1=3-5=-2,a20=3*20-5=55.
s20=(a1+a20)*20/2=53*10=530,
sn=(a1+an)*n/2=(-2+3n-5)*n/2=(3n-7)*n/2

sn=(a1+an)n/2=(-2+3n-5)n/2=(3n-7)n/2
s20=530