Question

高一数列 在等差数列{an}中,a3+a4+a5=84,a9=73.

Answer 1

an=a1+(n-1)d

a3+a4+a5=84

3a1+9d=84 (1)

a9=73

a1+8d=73 (2)

3(2)-(1)

15d=135

d=9

a1=1

an = 1+ (n-1)9 = 9n-8

(2)

m=1

9n-8 > 9

n=2

9n-8 < 92

9n 18

n=3

9n-8 < 184

n=21

b2= 19 ; n=3,4,...,21

m=3

9n-8 > 27

n=4

9n-8