Choosen As Best Answer

S10 = (2a + 9d) × 10 ÷ 2 = 265
S20 = (2a + 19d) × 20 ÷ 2 = 1030
整理後:
2a + 9d = 53 ...(1)
2a + 19d = 103 ...(2)

=> a = 4, d = 5 #

\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/

S19 = a1 + a2 + a3 + ... + a18 + a19

又 a1 + a19 = a2 + a18 = a3 + a17 = ... = a9 + a11 = 中間項 a10 的兩倍

a3 + a17 = 50 => a10 = 25

50 × 9 + 25 = 475 #

(看不懂我再補充)

CC

謝謝你~

Post A Comment
News
【IG直播懶人包】十大科目筆記方法分享
【開學必讀】準高中、準大學生必知二三事
【筆記方法】國文筆記怎麼做,三個超強筆記方法大公開