Question

coco

請問第2題怎麼解，謝謝

1 xy
(A) 2宮       吧)還8儿       (@) 4        (DU IM

(2若一等差相數的首項為~ ,且其前9項的和比前6項的和大 69,則此等差級數的公差為何.

 

 

 

 

 

(A) 3                     (B) 4                     (C) 5                     (D) 6

四| 2由泄條        入

Answer 1

✨ Jawaban Terbaik ✨

Ny-Ålesund

首項 = a = -5

S9 - S6 = a7 + a8 + a9
= a+6d + a+7d + a+8d
= 3a + 21d = 69
21d = 69 - 3a = 69 - 3 × (-5) = 84
=> d = 4 #

coco

了解了，謝謝你