Diket barisan aritmatika suku ke4 dan suku ke6 berturut-turut adalah 7 dan 23 maka tentukanla suku ke20

Mapel Matematika
Bab Baris dan Deret

U4 = 7
U6 = 23

a + 3b = 7
a + 5b = 23
a + 3b + 2b = 23
7 + 2b = 23
2b = 16
b = 8

a + 3b = 7
a + 3(8) = 7
a + 24 = 7
a = -17

U20 = a + 19b = -17 + 19(8) = -17 + 152 = 135

U4 = 7    →  a + 3b = 7 
U6 = 23  →  a + 5b           = 23
                    a + (3b + 2b) = 23
                    (a + 3b) + 2b = 23
                              7 + 2b  = 23
                                    2b  = 16
                                      b  =  8
a + 3b   = 7
a + 3(8) = 7
a + 24   = 7
          a = - 17

U20 = a + 19b
        = -17 + 19(8)
        = 135