Question

1.已知a1=1,a7=-17,求公差d与a11

Answer 1

1.a7=a1+6d=1+6d=-17 d=-3 a11=a1+10d=1+(-30)=-29

2.a1=a2-d=-1 an=a1+(n-1)d=-1+(n-1)3=20 n=8 sn=(a1+an)n/2=76 3.

3.a7=a4+3d 3d=-28 d=-28/3 a4=a1+3d a1=63

4.a1=1 a5=16 a5=a1*q的（N-1)次方 q=2 a7=2的6次方=64