Deret aritmatika diket U3=8 dan U7=20 suku ke 20 pd deret tersebut adalah

U3= a+2b =8
U7= a+6b =20
---------------- -
-4b = -12 , b = 3
a+2b = 8
a+6 =8
a=2

U20= a+19b = 2+ 19.3 =59

*U₃ = 8
a + (3-1)b = 8
a + 2b = 8

*U₇ =  20
a + (7-1)b = 20
a + 6b = 20

eliminasi
a + 2b = 8
a + 6b = 20  -
     -4b = -12
        b = 3

sub. b =3 kedlam pers 1
a + 2b = 8
a + 2.3 = 8
a = 8 - 6
a = 2

* jadi
U₂₀ = a + (20-1) b
       = 2 + 19.3
      = 2 + 57
     = 59