1 (1) 初項-6, 公差5の等差数列の,初
(2)次の等差数列の和を求めよ。
85, 78, 71, ****, 43/
(1) Sn = ±n { 20+ (n-1)d}
=
10 × (2.-6)
1/2×20{20+19×5}
(+)\xh (a+b)
xn
=
19
95
830
1150
//xn (85+43) = ±nx (2₤24
20
115
85
43
TLA
= 64n
1/2.7
±·7 (85+43)=448
85+ (n-1)-(-9)=43
-74192=43 n=7