Question

1.已知a1=1,a7=-17,求公差d与a11

Answer 1

1、a7=a1+6d=1+6d=-17,所以d=-3,a11=a1+10d=1-30=-29

2、an=a2+(n-2)d=2+(n-2)*3=3n-4=20,所以n=8,a1=a2-d=-1,sn=n*a1+n(n-1)d/2=-8+84=76

3、a4=35,a7=7,又a7=a4+3d=35+3d=7,所以d=-28/3,a4=a1+3d=a1-28=35,所以a1=63

4、a1=1,a5=16,又a5=a1*q^4=q^4=16,所以q=2,a7=a1*q^6=2^6=64