Question

Mohon bantuannya kaka ttg aritmetika

Answer 1

Un = a+(n-1)b

U2 = a + b = 9
U4+U7
= a+3b+a+6b
= 2a+9b
= 39

2a + 9b = 39  X1  
  a +   b = 9   X2

2a + 9b = 39
2a + 2b = 18
____________  _
        7b = 21
          b = 21/7 = 3
2a + 9b  = 39
2a + 9(3)= 39
2a + 27  = 39
2a          = 39 - 27
2a          = 12
  a          = 12/2 = 6

U18 = a+17b
U18 = 6 + 17(3)
U18 = 6 + 51
U18 = 57

Jawabannya d.57