Question

Without adding find the sum of 1+3+5+7+9+11+13+15+17+19+21.​

Answer 1

Answer: 121

Explanation:
Total consecutive odd numbers = 11
Thus n = 11
Therefore sum = n*n
= 11*11= 121




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Answer 2

it is an A.P. as number are in series with an common difference of 2

here a = 1

d = 2

an= 21

n= 11

Sn= n/2(2a +(n-1) d)

= 11/2(2×1 +(11-1)2)

= 11/2(2+20)

= 11/2× 22

= 11×11

= 121

so the sum of 1+3+5+7+9+11+13+15+17+19+21 is equals to 121