Jawabanmu

2014-05-04T12:31:36+07:00
U5 =18
U11=42

b =  \frac{42-18}{11-5} = 4

U5 = U1 + (n-1)b
18 = U1 + ( 5-1)4
18 = U1 + 16
18-16 = U1
2 = U1

Sn =  \frac{n}{2} ( 2.U1 +(n-1)b
S30=  \frac{30}{2} ( 2.2 + (30-1)4
      = 15(4+ (29.4)
      = 15(116)
      = 1740

mian klo salah
2014-05-04T12:58:30+07:00
Rumus Un = a+(n-1)b dimsukin aja, hasilnya nanti

a + 10b = 42
a + 4b =  18
____________ _
        6b = 24 ==> Terus dieliminasi
          b = 24/6
          b = 4

Metode sititusi

a + 4(4) = 18
a + 16 = 18
a = 18-16
a = 2

S30 = 1/2n((2a + (n-1)b)
       = 1/2 x 30( 2 x 2 + (30-1) 4)
       =15 ( 4 + (29 x 4))
       = 15 ( 4 + 116)
      = 15 (120)
      = 1800