Question

在等差数列中,a2+a5+a8=9,a3a5a7=-21,则a11的值

Answer 1

a2+a5+a8=9

3a5=9

a5=3

a3a5a7=-21

a3a7=-7

(a5-2d)(a5+2d)=-7

a5^2-4d^2=-7

9-4d^2=-7

d^2=4

d=±2

a11=a5+6d=3+12=15

a11=a5+6d=3-12=-9