Question

Suatu barisan aritmatika. diket U5=15 dan U12=43, nilai U21 adlh...
a. 69
b. 76
c. 79
d. 83

jbntuin dg crnya ya!

Answer 1

Un = a + (n-1)b
U5 = a + 4b
15 = a + 4b ..........(1)

U12 = a + 11b
43 = a + 11b .........(2)

a + 4b = 15
a + 11b = 43
------------------- —
-7b = -28
b = 4

a + 4b = 15
a + 16 = 15
a = -1

Un = a + (n-1)b
U21 = -1 + 20 . 4
U21 = 79

Answer 2

U5=a+(5-1)b=15
      a+4b=15.......(1)
u12=a+(12-1)b=43
         a+11b=43.....(2)

a+4b=15
a+11b=43 -
-7b = -28
   b = 4

a+4b=15
a+4(4)=15
a=15-16
a=-1

u21=a+(n-1)b
      =-1+(21-1)4
      = -1+(20)4
      = -1+80
      = 79