Question

Ini gimana ya tolong bantuin pakai cara​

Answer 1

Penjelasan dengan langkah-langkah:

U₃ = 15

U₃ = a + 2b

a + 2b = 15.............(1)

U₇ = 31

U₇ = a + 6b

a + 6b = 31.............(2)

eliminasi (2) dan (1)

a + 6b = 31

a + 2b = 15

---------------- --

0 + 4b = 16

b = 16/5

b = 4 .......(beda)

a + 2b = 15

a + 2(4) = 15

a = 15 - 8

a = 7.......(suku pertama )

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

jumlah 80 suku pertama (S₈₀)

S₈₀ = 80/2[ 2(7) + (80 - 1)(4) ]

S₈₀ = 80/2[ 14 + (79)(4) ]

S₈₀ = 40 [ 14 + 316]

S₈₀ = 40 x 330

S₈₀ = 13.200

Answer 2

Jawab:

a. beda

= 7 - 3 = 4

= (u7-u3) : 4

=  (31 - 15) : 4

= 16 : 4

= 4

b. suku pertama

u3= a + (n-1)b

15 = a + (3-1)4

15 = a + 8

a = 15 - 8

a = 7

c. jumlah 80 suku pertama

u80 = a + (n-1)b

u80 = 7 + (80-1)4

u80 = 7 + 316

u80 = 323

s80 = n (a +u80)

s80 = x 80 (7 +323)

s80 = 40 (330)

s80 = 13.200