Jawabanmu

2014-04-15T20:57:55+07:00
U3 = 42 
U7 = 26 
U15 = ? 

Un = a + (n-1)b 
U7 = a + 2b 
U12 = a + 6b 

a + 2b = 42
a + 6b = 26 
-------------------- - 
..... -4b = -16
........ b = 4 

a + 2(4) = 42
a + 24 = 42
a = 20

U15 = a + 14b = 20 + 14(4) = 56 + 32 =  88
1 5 1
2014-04-15T21:08:51+07:00
A+2b=42
a+6b=26
------------- -
   -4b=16
      b=16/-4
        =-4
a+2.-4=42
      a  =42+8
          =50
S15 = 15/2 (2.50 + (14)-4)
       = 7,5 (100 -56)
       = 7,5 x 44
       = 330
Jawabannya 330

Semoga Membantu :D
thanks , jawabanya ada (y)
ok, sma-sama :D