Question

Tentukan jumlah dari deret aritmetika 90+86+82+78+74....+10

Answer 1

A = 90
b = 86 - 90 = -4
Un = 10
a + (n-1)b = 10
90 + (n - 1) -4 = 10
-4 (n-1) = -80
n- 1 = 20
n = 21
Sn = n/2 (2a + (n-1)b)
S21 = 21/2 (2 (90) + 20 (-4))
       = 21/2 (180 - 80)
     = 21/2 (100)
     = 21 (50)
     = 1.050