Question

Show that the sum of an AP whose first term is a, the second term b and the last term c, is equal to `((a + c)(b + c - 2a))/(2(b - a))`

Answer 1

Given that, the AP is a, b, c

Here, first term = a,

Common difference = b – a

And last term, l = a_n = c

∵ a_n = l = a + (n – 1)d

⇒ c = a + (n – 1)(b – a)

⇒ (n – 1) = `(c - a)/(b - a)`

n = `(c - a)/(b - a) + 1`

⇒ n = `(c - a + b - a)/(b - a)`

= `(c + b - 2a)/(b - a)`    ...(i)

∴ Sum of an AP,

S_n = `n/2 [2a + (n - 1)d]`

= `((b + c - 2a))/(2(b - a)) [2a + {(b + c - 2a)/(b - a) - 1}(b - a)]`

= `((b + c - 2a))/(2(b - a))[2a + (c - a)/(b - a) * (b - a)]`

= `((b + c - 2a))/(2(b - a)) (2a + c - a)`

= `((b + c - 2a))/(2(b - a)) * (a + c)`

Hence proved.