Jawabanmu

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2014-05-02T19:29:58+07:00
U8=20
U10=12
S10?

U8=a+7b=20
U10=a+9b=12
         -2b=8
           b=-4
a+9b=12
a-36=12
a=12+36=48

s10=n/2 (2a+(n-1)b)
=5 (96-36)
=5 (60)
=300