Question

44

求解

(
K適康版B卷
) 13. 已知一等差數列第6項與第 11 項的比為2:
3,則此等差數列第8項與第 12 項的比為何?
(A) 2:3
(B) 3:4
(C) 4:5
(D) 5:6

Answer 1

chin

a1:a11=2:3
=>al+5d:a1+10d=2:3
內項乘積等於外項乘積=>2a1+20d=3a1+15d
=>a1=5d

a8:a12
=>a1+7d:a1+11d
=>5d+7d:5d+11d = 12d:16d = 3:4
=>a8:a12 = 3:4