barisan aritmatika mohon jawab

1.
b = (38-18)/(7-3) = 20/4 = 5
u3 = a + 2b = 18
a = 18 -2x5 = 8

S24 = 24/2 x (2x8 + 23x5)
= 12 x (16+115)
= 12 x 131
= 1.572

2.
b = (13-7)/(8-5) = 6/3 = 2
u5 = a + 4b = 7
a = 7 - 4x2 = -1

U20 = -1 + 19x2 = 37

3.
b = (30-14)/(7-3) = 16/4 = 4
u3 = a + 2b = 14
a = 14 - 2x4 = 6
S30 = 30/2 x (2x6 + 29x4)
= 15 x (12 + 116)
= 15 x 128
= 1.920

4.
Un = 2n² -5
U4 = 2(4)² -5 = 32-5 = 27
U5 = 2(5)² -5 = 50-5 = 45
U4 + U5 ============== +
= 72

Mapel : Matematika
Materi : Barisan dan Deret
Kelas : 9

[Nomor 1]
=========
⏩Beda Barisan

U7 - U3 = 38 - 18
7b - 3b = 20
4b = 20
b = 20/4
b = 5

⏩Suku Pertama

U3 = 18
a + 2b = 18
a + 10 = 18
a = 18 - 10
a = 8

⏩Jumlah 24 Suku Pertama

Sn = n/2 [2a + (n - 1)b]

S24 = 24/2 [2(8) + 23(5)]
S24 = 12 [16 + 115]
S24 = 12 x 131
S24 = 1572
============================

[Nomor 2]
==========
⏩Beda Barisan

U8 - U5 = 13 - 7
8b - 5b = 6
3b = 6
b = 6/3
b = 2

⏩Suku Pertama

U5 = 7
a + 4b = 7
a + 8 = 7
a = -1

⏩Suku Ke - 20

Un = a + (n - 1)b

U20 = -1 + 19(2)
U20 = -1 + 38
U20 = 37
============================

[Nomor 3]
=========
⏩Beda Barisan

U7 - U3 = 30 - 14
7b - 3b = 16
4b = 16
b = 16/4
b = 4

⏩Suku Pertama

U3 = 14
a + 2b = 14
a + 8 = 14
a = 14 - 8
a = 6

⏩Jumlah 30 Suku Pertama

S30 = 15 [2(6) + 29(4)]
S30 = 15 [12 + 116]
S30 = 15 x 128
S30 = 1920
============================

[Nomor 4]
==========
Un = 2n² - 5

= U4 + U5
= [2(4)² - 5] + [ 2(5)² - 5]
= [32 - 5] + [50 - 5]
= 27 + 45
= 72