deret aritmatika, u6=17 u10=33,jml 30 suku pertama deret?

U10=a+9b=33
U 6 =a+5b=17
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4b=16
b=16/4
b=4

a+5b=17
a+5.4=17
a=17-20
a=-3

Sn=½n(2a+b(n-1))
S30=½.30(2(-3)+4.29)
S30=15(110)
S30=1650