diketahui deret aritmatika dengan u5=17 dan u8=26 hitunglah s22

a+4b = 17 
a+7b = 26

(a+4b)+3b = 26
17 + 3b = 26
3b = 9
b=3
a= 5

a+21b
5 + (21 x 3) = 68

u5 = 17
u8 = 26

u5 = a + 4b = 17
u8 = a + 7b = 26

eliminasi
b = 3
a = 5

u22 = 5 + (22-1)3 = 68

S22 = 22/2 (5 + 68) = 803