Jawabanmu

2014-03-19T19:15:40+07:00
U7 = a +6b = 103
u10 = a +9B = 88.
dieliminasi b = -5, a= 133

Sn20 = 20/2( 133.2+(20-1)-5)
= 10(266-95)
= 10 x 171
= 1710. maaf kalau salah
2014-03-19T19:17:14+07:00
u7=103 \\a+6b=103 \\  \\ u10=88 \\ a+9b=88 \\  \\b=-5 \\a=133 \\ u20=133+19(-5) \\=133-95 \\ =38 \\  \\s20=20/2(133+38) \\ =10(171) \\=1710
kalau b = -5 itu dari mana ?
eliminasi x
a+6b=103-a+9b=88