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a+2r+a+5r=29
a+4r+a+10r=50
2a+7r=29 ⇒2a=29-7r
2a+14r=50
29-7r+14r=50
7r=21
r=3
2a=29-7*3
2a=8
a1=4
Odp. a1=4, r=3.
2.
a1=16=2^4
q=8/16=1/2
an=1/32=2^{-5}
an=2^4*(1/2)^{n-1}=2^{-5}
n=10
Sn=16*(1-(1/2)^{10})/(1-1/2)=16*2*(1-1/1024)=32*1023/1024=
1023/32