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2-3-3-=4√5-9√5+12√5-5√5=2√5
an=4n+9
a1=4*1+9=13
a2=4*2+9=17
a3=4*3+9=21
a18=4*18+9=81
Sn=((a1+an)/2)*n
S18=((13+81)/2 )*18=846.