Pake cara ya kawan :) Yg nomor 1 itu a dan batin aja ya

itu no 1 a.. yg b sabar ya

rumus yang digunakan
[tex]sn = \frac{n}{2} (2a + (n - 1)b)[/tex]

No 1 A
diketahui : a = 80

b = U2 - U1
= 70 - 80
= -10

[tex]s20 = \frac{20}{2} (2.80 + (20 - 1) - 10)[/tex]
[tex]s20 = 10(160 + 19 \times - 10)[/tex]
[tex]s20 = 10(160 + - 190)[/tex]
S20 = 10(-30)
S20 = -300

No 1 B
diketahui : a = 10

b = U2-U1
= 5 - 10
= -5

[tex]s20 = \frac{20}{2} (2.10 + (20 - 1) - 5[/tex]
S20 = 10(20 + (19)-5)
S20 = 10(20 + -95)
S20 = 10(-75)
S20 = -750