Question

the fifth term of an ap is 10 find the sum of its first. 9 terms

Answer 1

Answer:

heya mate there's your answer

let a be first term and d be the common difference of the AP

given 5th term a5=10

a5=10

a+(n-1)d=nth term(an)

therefore a5=a+(5-1)d=10

a+4d=10

now,Sum of n terms in an AP is

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

Now Sum of first 9 terms in the AP,

S9=9/2(2a+(9-1)d)

=9/2(2a+8d)

=9/2 (2)(a+4d)

=9(a+4d)

=9(10)

=90.

So sum of first 9 terms of an APis 90

hope it helps you

Thank you