Jawabanmu

2014-03-28T09:21:10+07:00
b = \frac{37 - 25}{10 - 7} =  \frac{12}{3} = 4
dari U7 = 25 didapat
a + 6b = 25
a + 6.4 = 25
a + 24 = 25
a = 1. sehingga
 S_{20} =  \frac{20}{2}(2.1 + 19. 4) = 10(2 + 76) = 10.78 = 780
2014-03-28T09:30:41+07:00
U7=a+6b=25
U10=a+9b=37
S20=...?
a+6b=25
a+9b=37-
-3b=-12      b=4

a+6(4)=25
a=1

 S_{n}= \frac{n}{2}(a+un)  atau  \frac{n}{2}(a+(a+ (n-1)b) )
 S_{20}= \frac{20}{2}(1+(1+19(4)))
=780