Question

Poinnya lumayan nii hehe
tlg dibantu ya nomor 7
pakai cara. trims:)

Answer 1

Un= 2n + 19
u1 = 2 + 19 = 21
u2= 4 + 19 = 23

a = 21
b = 23-21= 2

a. sn = n/2 ( 2a + (n-1)b
        = n/2 (2x21 + (n-1)2
       = n/2 (42 + 2n - 2)
       = n/2 (40 + 2n)
      = 20n + n²

b. S20 = 20x20 + 20²
            = 400 + 400
            = 800