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2014-01-07T19:37:55+02:00
A3=12=>a1+2*r=12=>a1=12-2*r
a6=30=>a1+5*r=30                 
=>  12-2*r+5*r=30=> 3*r=18=>r=6
a1=12-2*6=0
a10=a1+9*r=0+54=54
5 4 5