find the sum of first 21 terms of an ap where the middle term is 20
Answers
Answered by
3
hope my answer will help you
Attachments:
saralogu4uin:
Answer wrong
so what is the right answer
can you tell me please
minnu is the answer right or wrong
Answered by
3
Let the first number be a
common difference be d
S21= ?
middle term T11=20
T11 =a+(11-1)d
20 = a+10d
a=20-10d
S21 = 21/2[2a+(21-1)d]
= 21/2[2(20+10d)+20d]
=21/2[40-20d+20d]
=21/2[40]
=420______answer
Similar questions