Question

sum of 1+3+5+7+---------+21​

Answer 1

Answer:

a=1

d=a2-a1

=3-1

=2

an=21

an=a+(n-1)d

21 =1+(n-1)(2)

20=(n-1)(2)

10=(n-1)

11=n

sn=n/2(a+an)

sn=11/2(1+21)

sn=11/2(22)

sn=11(11)

sn=121 -Ans

Answer 2

Step-by-step explanation:

An=a+(n-1)d

21= 1+(n-1)2

21= 1+2n-2

21+1= 2n

n= 22/2

n=11

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

S11= 11/2(2(1)+(11-1)2)

= 11/2 ( 22)

= 11/2×22

= 121