The sum of first 20 odd counting number is???
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Answer:
counting numbers start from 1 onwards
a = 1 and d = 2 [ difference of 2 between two consecutive odd numbers ]
S₂₀ = n/2[2a + (n - 1)d]
S₂₀ = 20/2 [ 2*1 + 19*2]
=> 10 [2 + 38]
=> 10*40
=> 400
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