等差数列中an中,已知S5=5,an-2=5,Sn=60等差数列中an中,已知S5=5,an-2=5,Sn=60,n=?

Question

等差数列中an中,已知S5=5,an-2=5,Sn=60
等差数列中an中,已知S5=5,an-2=5,Sn=60,n=?

Answer 1

s5=5a1+10d=5
∴a3=a1+2d=1
sn=[a3+a(n-2)]n/2=60 ——[an+a1=a3+a(n-2)]
=(1+5)n/2=60
∴n=20