Jawabanmu

2014-04-16T21:20:55+07:00
(U3 = a + 2b = 14) - (U7 = a + 6b = 26)
4b = 12
b = 3, a = 8
S18 = 18/2 (2.8 + 17.3)
  = 9 (16 + 51)
  = 9.67 = 603

2014-04-16T21:20:57+07:00
A+2b = 14
a+6b=26, pakai substitusi, jadi b=3, dan a =8

jumlah 18 suku pertama = n/2 (2a + (n-1).b)
=18/2 (2.8+(18-1).3)
=9 (16+ (17.3)
=9 (16+51)
=603