Question

Jumlah 3 suku awal bilangan=51,jika u8+u9=60,maka jumlah 31 bilangan awal tsb adalah?pake cara ya

Answer 1

S3 = a + a+b + a + 2b = 51
     = 3a + 3b = 51..> kali 2

u8+u9 = (a+7b) + (a+ 8b) = 60
         = 2a + 15b = 60...> kali 3

6a +   6b  = 102
6a + 45b = 180 -
     -39b = -78
b = -78/-39 = 2

2a + 15b = 60
2a + 30 = 60
2a = 30
a= 15

S31 = 1/2n( 2a + (n-1)b
       = 1/2*31 ( 2*15 + (31-1)2
      = 15,5 (30 + 60)
      = 15,5 (90)
     = 1.395