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a3=a1 + 2r
a5= a1 + 4r
10=a1 + 2r/*(-1)
18= a1 + 4r
-10=-a1 - 2r
18= a1 + 4r
8=2r
r=4
10=a1+8
a1=2
an=a1+(n-1)r
an=2+(20-1)4
an=78
s20=2+78/2*20=800