Question

If the first, second and last term of an A.P. are a,b and 2a respectively find sum ?​

Answer 1

Hera a1= a

a2= b

an= 2a 9

now common difference is

d= a2-a1

d= b-a

therefore d= b-a.

we know that

an= a +(n-1)d= 2a

a +(n-1) b-a= 2a

(n-1)b-a=2a-a

(n-1) b= a+a

(n-1)= 2a/b

n= 2a/b+1

n= 2a+b/b.

now putting the value for finding sum

Sn = n/2(a+an)

now see the attachment

not truly sure about answer...I tried this for first time......