pada barisan aritmetika U5=16 dan U11= 34 jumlah 40 suku pertama barisan bilangan tersebut adalah

u11 = a + 10b = 34
u5 = a + 4b = 16
--------------------------- -
              6b = 18
                b = 3

a + 4b = 16
a + 4.3 = 16
a + 12 = 16
a = 16 - 12 = 4

u40 = a + 39b
        = 4 + 39.3
        = 4 + 117
        = 121

s40 = 1/2 n ( a + U40)
       = 1/2. 40 ( 4 + 121)
       = 20.125 = 2500 

A).U5:16 ➡a+4b :16
U11:34➡a+10b:34
_______ _
-6b:-18
b:-18/-6
b:3
B).a+4b :16
a+4.3 :16
a+12. :16
a. :16-12
a. :4
C).Un: a+(n-1)b
U40:4+(40-1)3
:4+39.3
:4+117
:121
D).4,7,10,13,16,19,22,25,28,31,34,37,40,43,46,49,52,55,58,61,64,67,70,73,76,79,82,85,88,91,94,97,100,103,106,109,112,115,118,121