1) d=a2–a1=13–20=–7
a7=a1+6d=20+6•(-7)=–22
2) d=a2–a1=–5+9=4
a8=a1+7d=–9+7•4=19
3) d=a2–a1=–4+7=3
2a1+5d
S6 = ----------- •6 = (2•(-7)+5•3)•3 = 3
2
4) q=b2/b1=–525/175=–3
b4=b1•q^3=175•(-27)=–4725
5) q=b2/b1=150/(-750)=–1/5
b1•(q^5–1) -750•(-1/3125–1)
S5 = --------------- = ----------------------- =
q–1 –4/5
750,24 750,24•5
= ---------- = -------------- = – 937,8
-4/5 –4
6) a5=a1+4d=–6,8+4•(–8,5)=–6,8–34
=–40,8
7) d=–4
c6=c1+5d=5+5•(-4)=–15
8) d=4
a6=a1+5d=3+5•4=23
9) d=–17
2a1+6d
S7 = ----------- • 7 = (a1+3d)•7 =
2
= (48+3•(-17))•7=–21
10) q=–3
b7=b1•q^6=–113•729=–82377
11) q=2
b7=b1•q^6=–2•64=–128
12) q=–4
b1•(q^5–1) –3•(–1025)
S5 = --------------- = ---------------- = –615
q–1 –5
13) d=a4–a3=–15+13=–2
x=a2=a1+d=–9–2=–11
14) q=b4/b3=–96/24=–4
x=b2=b1•q=1,5•(-4)=–6
15) b2=–2•(–1/6)=1/3
b3=–2•3=–6
b4=–2•(–1/6)=1/3
b5=–2•3=–6
16) {a1+5d=–7,8
{a1+18d=–10,4
–13d=2,6
d=–0,2