Tolong dong, sekarang​

Diket

U5 = 15

U8 = 21

Ditanya = S15..?

Jawab

U8 = a + 7b = 21

U5 = a + 4b = 15

——————– –

3b = 6

b = 6/3

b = 2

a + 4b = 15

a + 4(2) = 15

a + 8 = 15

a = 15 – 8

a = 7

Sn = n/2 (2a + (n – 1)b)

S15 = 15/2 (2(7) + (15 – 1)2)

S15 = 15/2 (14 + 28)

S15 = 15/2 (42)

S15 = 15 (21)

S15 = 315 (C)

semoga membantu

S-5 =15==>a+4b =15

S-8 =21==>a+7b =21

-3b=-6

b=2

a+4b =15

a+4(2)=15

a. =15-8

a. =7

S-15 =n/2(a+(n-1)).b

. =15/2(7+(15-1))2

=15×21

=315

maaf kalo salah kak..