Question

1.di ketahui : U4=7
U6+U8=23
di tanya : sn
2.di ketahui : s4=17
s8=58
di tanya: s1

Answer 1

U4 = a + 3b = 7
U6 + U8 = a + 5b + a + 7b =2a + 12b = 23
Persamaan:
  a +   3b =  7
2a + 12b = 23
-----------------------
2a +  6b = 14
2a + 12b = 23
----------------------- (dikurangi)
-6b = -9
b = -9/-6 = 3/2
maka a = a + 3b = 7
a + 3(3/2) = 7
a + 9/2 = 7
a = 7 - 9/2
a = 14/2 - 9/2
a = 5/2 
Sn = n/2 (2a + (n-1)b) = n/2 (2(5/2) + (n - 1) 3/2) = n/2 (5 + 3/2n - 3/2)
= n/2 (10/2 - 3/2 + 3/2n)
= n/2 (7/2 + 3/2n) = 7n/4 +3n^2/4 = n (7/4 + 3n/4)