Jawabanmu

2014-02-26T15:28:48+07:00
1. U2=a+b=20. U3=a+2b=8. B=-12. a+b=20. a=32. S40=20*-404=-8080. 2. Un=492. 492=402+(n-1)3. n=31. S31=31/2*894=13857
2014-02-26T15:38:41+07:00
Un = a + (n-1)b

U3 = a + 2b = 8
U2 = a + b = 20
dieliminasi
a+b = -12
b = -12

U3 = a +2b = 8
U3 = a + 2.-12 = 8
U3 = a - 24 = 8
a = 8+24
a = 32

U1=a + (n-1)b
U1= 32+(1-1)-12
U1 = 32+0.-12
U1=32+0
U1= 32

Sn = 1/2n ( 2a + (n-1)b)
S40 =1/2.40 (2.32 + (40-1)-12
S40=20(64+39.-12)
S40=20(34+458)
S40=20.492
S40=17880