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let s=20+21+--------------------22008
let s1=21+22+---------------22008
s1 is a geometric progression with a=2,r=2 & n=2008
so, s1=a(rn-1)/r-1
s1=2(22008-1)/2-1 = 22009-2
now, s=20+ s1=1+22009-2=22009-1.
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