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प्रश्न
The sum of first 20 odd natural numbers is:
विकल्प
A. 100
B. 210
C. 400
D. 420
उत्तर
Odd natural numbers are in the pattern 1, 3, 5, 7, 9…,.
These numbers form an A.P. where a = 1, d = 3 − 1 = 2
We know that, `S_n=n/2[2a+(n-1)d]`
`thereforeS_20=20/2[2xx1+(20-1)xx2]`
`=20/2[2+19xx2]`
`=10[2+38]`
`=10xx40`
`=400`
Thus, the sum of first 20 odd natural numbers is 400.
Hence, the correct answer is C.
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